fix(api): generate /ID for all saves if document lacks one

PDFs without an /ID array cause signature validation issues in Adobe
Reader. Now the library ensures all saved PDFs have an /ID by either
preserving the existing one or generating a new 16-byte random ID.

Author: Mythie

src/api/pdf.test.ts

@@ -375,6 +375,69 @@ describe("PDF", () => {
       expect(appendedSection).toContain("trailer");
       expect(appendedSection).toContain("/ID");
     });
+
+    it("generates /ID if document lacks one (new document)", async () => {
+      // Create a new document which won't have an /ID
+      const pdf = PDF.create();
+
+      pdf.addPage();
+
+      // Full save should generate an /ID
+      const saved = await pdf.save();
+      const text = new TextDecoder().decode(saved);
+
+      // The trailer should include a generated /ID array
+      expect(text).toContain("/ID");
+
+      // Verify the ID format: should be two identical 16-byte hex strings
+      const idMatch = text.match(/\/ID\s*\[\s*<([a-fA-F0-9]+)>\s*<([a-fA-F0-9]+)>/);
+
+      expect(idMatch).not.toBeNull();
+
+      const [, id1, id2] = idMatch!;
+
+      // Both IDs should be 32 hex chars (16 bytes)
+      expect(id1.length).toBe(32);
+      expect(id2.length).toBe(32);
+      // Both values should be identical for a newly generated ID
+      expect(id1).toBe(id2);
+    });
+
+    it("generates /ID if document lacks one (loaded PDF)", async () => {
+      // pdf-overlay-page-1.pdf is a real PDF without an /ID
+      const bytes = await loadFixture("scenarios", "pdf-overlay-page-1.pdf");
+      const originalText = new TextDecoder().decode(bytes);
+
+      // Verify the original document lacks an /ID
+      expect(originalText).not.toContain("/ID");
+
+      const pdf = await PDF.load(bytes);
+
+      // Modify to trigger a save
+      const catalog = pdf.getCatalog();
+
+      catalog?.set("ModDate", PdfString.fromString("D:20240101"));
+
+      // Save should generate an /ID
+      const saved = await pdf.save();
+      const text = new TextDecoder().decode(saved);
+
+      // The trailer should include a generated /ID array
+      expect(text).toContain("/ID");
+
+      // Verify the ID format: should be two identical 16-byte hex strings
+      const idMatch = text.match(/\/ID\s*\[\s*<([a-fA-F0-9]+)>\s*<([a-fA-F0-9]+)>/);
+
+      expect(idMatch).not.toBeNull();
+
+      const [, id1, id2] = idMatch!;
+
+      // Both IDs should be 32 hex chars (16 bytes)
+      expect(id1.length).toBe(32);
+      expect(id2.length).toBe(32);
+      // Both values should be identical for a newly generated ID
+      expect(id1).toBe(id2);
+    });
   });
 
   describe("copyPagesFrom", () => {

src/api/pdf.ts

@@ -38,6 +38,7 @@ import type { StandardSecurityHandler } from "#src/security/standard-handler.ts"
 import type { SignOptions, SignResult } from "#src/signatures/types";
 import type { FindTextOptions, PageText, TextMatch } from "#src/text/types";
 import { writeComplete, writeIncremental } from "#src/writer/pdf-writer";
+import { randomBytes } from "@noble/ciphers/utils.js";
 import { deflate } from "pako";
 
 import { PDFAttachments } from "./pdf-attachments";
@@ -2639,19 +2640,26 @@ export class PDF {
     }
     // Note: action === "remove" means no encrypt dict (decrypted on load, written without encryption)
 
-    // For incremental saves, preserve the document /ID if not already set
-    // This is required for digital signatures to validate properly
-    if (useIncremental && !fileId) {
+    // Ensure document has an /ID (required for signatures, recommended for all PDFs)
+    if (!fileId) {
       const idArray = this.ctx.info.trailer.getArray("ID");
 
       if (idArray && idArray.length >= 2) {
+        // Preserve existing ID
         const id1 = idArray.at(0);
         const id2 = idArray.at(1);
 
         if (id1 instanceof PdfString && id2 instanceof PdfString) {
           fileId = [id1.bytes, id2.bytes];
         }
       }
+
+      // Generate new ID if document doesn't have one
+      if (!fileId) {
+        const newId = randomBytes(16);
+
+        fileId = [newId, newId];
+      }
     }
 
     // For incremental saves, use the same XRef format as the original document