diff --git a/src/__tests__/convert.test.ts b/src/__tests__/convert.test.ts
index 5cc7c1c..507107b 100644
--- a/src/__tests__/convert.test.ts
+++ b/src/__tests__/convert.test.ts
@@ -182,3 +182,66 @@ describe("POST /v1/convert/url", () => {
     expect(res.headers["content-type"]).toMatch(/application\/pdf/);
   });
 });
+
+describe("PDF option validation (all endpoints)", () => {
+  const endpoints = [
+    { path: "/v1/convert/html", body: { html: "<h1>Hi</h1>" } },
+    { path: "/v1/convert/markdown", body: { markdown: "# Hi" } },
+  ];
+
+  for (const { path, body } of endpoints) {
+    it(`${path} returns 400 for invalid scale`, async () => {
+      const res = await request(app)
+        .post(path)
+        .set("content-type", "application/json")
+        .send({ ...body, scale: 5 });
+      expect(res.status).toBe(400);
+      expect(res.body.error).toContain("scale");
+    });
+
+    it(`${path} returns 400 for invalid format`, async () => {
+      const res = await request(app)
+        .post(path)
+        .set("content-type", "application/json")
+        .send({ ...body, format: "B5" });
+      expect(res.status).toBe(400);
+      expect(res.body.error).toContain("format");
+    });
+
+    it(`${path} returns 400 for non-boolean landscape`, async () => {
+      const res = await request(app)
+        .post(path)
+        .set("content-type", "application/json")
+        .send({ ...body, landscape: "yes" });
+      expect(res.status).toBe(400);
+      expect(res.body.error).toContain("landscape");
+    });
+
+    it(`${path} returns 400 for invalid pageRanges`, async () => {
+      const res = await request(app)
+        .post(path)
+        .set("content-type", "application/json")
+        .send({ ...body, pageRanges: "abc" });
+      expect(res.status).toBe(400);
+      expect(res.body.error).toContain("pageRanges");
+    });
+
+    it(`${path} returns 400 for invalid margin`, async () => {
+      const res = await request(app)
+        .post(path)
+        .set("content-type", "application/json")
+        .send({ ...body, margin: "1cm" });
+      expect(res.status).toBe(400);
+      expect(res.body.error).toContain("margin");
+    });
+  }
+
+  it("/v1/convert/url returns 400 for invalid scale", async () => {
+    const res = await request(app)
+      .post("/v1/convert/url")
+      .set("content-type", "application/json")
+      .send({ url: "https://example.com", scale: 5 });
+    expect(res.status).toBe(400);
+    expect(res.body.error).toContain("scale");
+  });
+});
diff --git a/src/__tests__/pdf-options.test.ts b/src/__tests__/pdf-options.test.ts
new file mode 100644
index 0000000..224886f
--- /dev/null
+++ b/src/__tests__/pdf-options.test.ts
@@ -0,0 +1,162 @@
+import { describe, it, expect } from "vitest";
+import { validatePdfOptions } from "../utils/pdf-options.js";
+
+describe("validatePdfOptions", () => {
+  // --- Happy path ---
+  it("accepts empty options", () => {
+    const result = validatePdfOptions({});
+    expect(result.valid).toBe(true);
+  });
+
+  it("accepts undefined", () => {
+    const result = validatePdfOptions(undefined as any);
+    expect(result.valid).toBe(true);
+  });
+
+  it("accepts all valid options together", () => {
+    const result = validatePdfOptions({
+      scale: 1.5,
+      format: "A4",
+      landscape: true,
+      printBackground: false,
+      displayHeaderFooter: true,
+      preferCSSPageSize: false,
+      width: "210mm",
+      height: "297mm",
+      margin: { top: "1cm", right: "1cm", bottom: "1cm", left: "1cm" },
+      pageRanges: "1-5",
+    });
+    expect(result.valid).toBe(true);
+    if (result.valid) {
+      expect(result.sanitized.scale).toBe(1.5);
+      expect(result.sanitized.format).toBe("A4");
+    }
+  });
+
+  // --- scale ---
+  describe("scale", () => {
+    it("accepts 0.1", () => {
+      expect(validatePdfOptions({ scale: 0.1 }).valid).toBe(true);
+    });
+    it("accepts 2.0", () => {
+      expect(validatePdfOptions({ scale: 2.0 }).valid).toBe(true);
+    });
+    it("rejects 0.05", () => {
+      const r = validatePdfOptions({ scale: 0.05 });
+      expect(r.valid).toBe(false);
+      if (!r.valid) expect(r.error).toContain("scale");
+    });
+    it("rejects 2.5", () => {
+      expect(validatePdfOptions({ scale: 2.5 }).valid).toBe(false);
+    });
+    it("rejects non-number", () => {
+      expect(validatePdfOptions({ scale: "big" as any }).valid).toBe(false);
+    });
+  });
+
+  // --- format ---
+  describe("format", () => {
+    const validFormats = ["Letter", "Legal", "Tabloid", "Ledger", "A0", "A1", "A2", "A3", "A4", "A5", "A6"];
+    for (const f of validFormats) {
+      it(`accepts ${f}`, () => {
+        expect(validatePdfOptions({ format: f }).valid).toBe(true);
+      });
+    }
+    it("accepts case-insensitive (a4)", () => {
+      const r = validatePdfOptions({ format: "a4" });
+      expect(r.valid).toBe(true);
+      if (r.valid) expect(r.sanitized.format).toBe("A4");
+    });
+    it("accepts case-insensitive (letter)", () => {
+      const r = validatePdfOptions({ format: "letter" });
+      expect(r.valid).toBe(true);
+      if (r.valid) expect(r.sanitized.format).toBe("Letter");
+    });
+    it("rejects invalid format", () => {
+      const r = validatePdfOptions({ format: "B5" });
+      expect(r.valid).toBe(false);
+      if (!r.valid) expect(r.error).toContain("format");
+    });
+  });
+
+  // --- booleans ---
+  for (const field of ["landscape", "printBackground", "displayHeaderFooter", "preferCSSPageSize"] as const) {
+    describe(field, () => {
+      it("accepts true", () => {
+        expect(validatePdfOptions({ [field]: true }).valid).toBe(true);
+      });
+      it("accepts false", () => {
+        expect(validatePdfOptions({ [field]: false }).valid).toBe(true);
+      });
+      it("rejects string", () => {
+        const r = validatePdfOptions({ [field]: "yes" as any });
+        expect(r.valid).toBe(false);
+        if (!r.valid) expect(r.error).toContain(field);
+      });
+      it("rejects number", () => {
+        expect(validatePdfOptions({ [field]: 1 as any }).valid).toBe(false);
+      });
+    });
+  }
+
+  // --- width/height ---
+  for (const field of ["width", "height"] as const) {
+    describe(field, () => {
+      it("accepts string", () => {
+        expect(validatePdfOptions({ [field]: "210mm" }).valid).toBe(true);
+      });
+      it("rejects number", () => {
+        expect(validatePdfOptions({ [field]: 210 as any }).valid).toBe(false);
+        const r = validatePdfOptions({ [field]: 210 as any });
+        if (!r.valid) expect(r.error).toContain(field);
+      });
+    });
+  }
+
+  // --- margin ---
+  describe("margin", () => {
+    it("accepts valid margin object", () => {
+      expect(validatePdfOptions({ margin: { top: "1cm", bottom: "2cm" } }).valid).toBe(true);
+    });
+    it("accepts empty margin object", () => {
+      expect(validatePdfOptions({ margin: {} }).valid).toBe(true);
+    });
+    it("rejects non-object margin", () => {
+      expect(validatePdfOptions({ margin: "1cm" as any }).valid).toBe(false);
+    });
+    it("rejects margin with non-string values", () => {
+      expect(validatePdfOptions({ margin: { top: 10 } as any }).valid).toBe(false);
+    });
+    it("rejects margin with unknown keys", () => {
+      expect(validatePdfOptions({ margin: { top: "1cm", padding: "2cm" } as any }).valid).toBe(false);
+    });
+  });
+
+  // --- pageRanges ---
+  describe("pageRanges", () => {
+    it("accepts '1-5'", () => {
+      expect(validatePdfOptions({ pageRanges: "1-5" }).valid).toBe(true);
+    });
+    it("accepts '1,3,5'", () => {
+      expect(validatePdfOptions({ pageRanges: "1,3,5" }).valid).toBe(true);
+    });
+    it("accepts '2-'", () => {
+      expect(validatePdfOptions({ pageRanges: "2-" }).valid).toBe(true);
+    });
+    it("accepts '1-3,5,7-9'", () => {
+      expect(validatePdfOptions({ pageRanges: "1-3,5,7-9" }).valid).toBe(true);
+    });
+    it("accepts single page '3'", () => {
+      expect(validatePdfOptions({ pageRanges: "3" }).valid).toBe(true);
+    });
+    it("rejects non-string", () => {
+      expect(validatePdfOptions({ pageRanges: 5 as any }).valid).toBe(false);
+    });
+    it("rejects invalid pattern", () => {
+      expect(validatePdfOptions({ pageRanges: "abc" }).valid).toBe(false);
+    });
+    it("rejects 'all'", () => {
+      expect(validatePdfOptions({ pageRanges: "all" }).valid).toBe(false);
+    });
+  });
+});
diff --git a/src/routes/convert.ts b/src/routes/convert.ts
index bdfb3eb..8e46885 100644
--- a/src/routes/convert.ts
+++ b/src/routes/convert.ts
@@ -6,6 +6,7 @@ import logger from "../services/logger.js";
 import { isPrivateIP } from "../utils/network.js";
 
 import { sanitizeFilename } from "../utils/sanitize.js";
+import { validatePdfOptions } from "../utils/pdf-options.js";
 
 export const convertRouter = Router();
 
@@ -94,6 +95,13 @@ convertRouter.post("/html", async (req: Request & { acquirePdfSlot?: () => Promi
       return;
     }
 
+    // Validate PDF options
+    const validation = validatePdfOptions(body);
+    if (!validation.valid) {
+      res.status(400).json({ error: validation.error });
+      return;
+    }
+
     // Acquire concurrency slot
     if (req.acquirePdfSlot) {
       await req.acquirePdfSlot();
@@ -203,6 +211,13 @@ convertRouter.post("/markdown", async (req: Request & { acquirePdfSlot?: () => P
       return;
     }
 
+    // Validate PDF options
+    const validation = validatePdfOptions(body);
+    if (!validation.valid) {
+      res.status(400).json({ error: validation.error });
+      return;
+    }
+
     // Acquire concurrency slot
     if (req.acquirePdfSlot) {
       await req.acquirePdfSlot();
@@ -339,6 +354,13 @@ convertRouter.post("/url", async (req: Request & { acquirePdfSlot?: () => Promis
       return;
     }
 
+    // Validate PDF options
+    const validation = validatePdfOptions(body);
+    if (!validation.valid) {
+      res.status(400).json({ error: validation.error });
+      return;
+    }
+
     // Acquire concurrency slot
     if (req.acquirePdfSlot) {
       await req.acquirePdfSlot();
diff --git a/src/utils/pdf-options.ts b/src/utils/pdf-options.ts
new file mode 100644
index 0000000..ec7fd05
--- /dev/null
+++ b/src/utils/pdf-options.ts
@@ -0,0 +1,88 @@
+const VALID_FORMATS = ["Letter", "Legal", "Tabloid", "Ledger", "A0", "A1", "A2", "A3", "A4", "A5", "A6"];
+const FORMAT_MAP = new Map(VALID_FORMATS.map(f => [f.toLowerCase(), f]));
+const PAGE_RANGES_RE = /^\d+(-\d*)?(\s*,\s*\d+(-\d*)?)*$/;
+const MARGIN_KEYS = new Set(["top", "right", "bottom", "left"]);
+
+type PdfInput = Record<string, any>;
+type ValidResult = { valid: true; sanitized: Record<string, any> };
+type InvalidResult = { valid: false; error: string };
+
+export function validatePdfOptions(opts: PdfInput): ValidResult | InvalidResult {
+  if (!opts || typeof opts !== "object") return { valid: true, sanitized: {} };
+
+  const sanitized: Record<string, any> = {};
+
+  // scale
+  if (opts.scale !== undefined) {
+    if (typeof opts.scale !== "number" || opts.scale < 0.1 || opts.scale > 2.0) {
+      return { valid: false, error: "scale must be a number between 0.1 and 2.0" };
+    }
+    sanitized.scale = opts.scale;
+  }
+
+  // format
+  if (opts.format !== undefined) {
+    if (typeof opts.format !== "string") {
+      return { valid: false, error: "format must be a string" };
+    }
+    const canonical = FORMAT_MAP.get(opts.format.toLowerCase());
+    if (!canonical) {
+      return { valid: false, error: `format must be one of: ${VALID_FORMATS.join(", ")}` };
+    }
+    sanitized.format = canonical;
+  }
+
+  // booleans
+  for (const field of ["landscape", "printBackground", "displayHeaderFooter", "preferCSSPageSize"]) {
+    if (opts[field] !== undefined) {
+      if (typeof opts[field] !== "boolean") {
+        return { valid: false, error: `${field} must be a boolean` };
+      }
+      sanitized[field] = opts[field];
+    }
+  }
+
+  // width/height
+  for (const field of ["width", "height"]) {
+    if (opts[field] !== undefined) {
+      if (typeof opts[field] !== "string") {
+        return { valid: false, error: `${field} must be a string (CSS dimension)` };
+      }
+      sanitized[field] = opts[field];
+    }
+  }
+
+  // margin
+  if (opts.margin !== undefined) {
+    if (typeof opts.margin !== "object" || opts.margin === null || Array.isArray(opts.margin)) {
+      return { valid: false, error: "margin must be an object with top/right/bottom/left string fields" };
+    }
+    for (const key of Object.keys(opts.margin)) {
+      if (!MARGIN_KEYS.has(key)) {
+        return { valid: false, error: `margin contains unknown key: ${key}` };
+      }
+      if (typeof opts.margin[key] !== "string") {
+        return { valid: false, error: `margin.${key} must be a string` };
+      }
+    }
+    sanitized.margin = { ...opts.margin };
+  }
+
+  // pageRanges
+  if (opts.pageRanges !== undefined) {
+    if (typeof opts.pageRanges !== "string") {
+      return { valid: false, error: "pageRanges must be a string" };
+    }
+    if (!PAGE_RANGES_RE.test(opts.pageRanges.trim())) {
+      return { valid: false, error: "pageRanges must match pattern like '1-5', '1,3,5', or '2-'" };
+    }
+    sanitized.pageRanges = opts.pageRanges;
+  }
+
+  // Pass through non-validated fields
+  for (const key of ["headerTemplate", "footerTemplate"]) {
+    if (opts[key] !== undefined) sanitized[key] = opts[key];
+  }
+
+  return { valid: true, sanitized };
+}