Add toPdf(): searchable PDF export with invisible RTR text overlay

.gitignore

@@ -2,6 +2,7 @@ lib/*
 node_modules/
 tests/output/*.png
 tests/output/*.jpg
+tests/output/*.pdf
 tests/output/*.cpuprofile
 *.DS_Store*
 .aider*

README.md

@@ -6,6 +6,23 @@ Ratta Supernote has often commented that the file-format is yet unstable and sho
 
 For some quick snippets, take a look at the [smoke tests](./tests/main.test.ts).
 
+### Generating searchable PDFs
+
+`toPdf` renders each page's raster image into a PDF page and overlays the recognized handwriting (RTR) text invisibly at the location it was written, so the PDF is searchable and words can be selected/copied from the image. See [tests/pdf.test.ts](./tests/pdf.test.ts) for a full example.
+
+```ts
+import { SupernoteX, toPdf } from 'supernote-typescript';
+
+const note = new SupernoteX(buffer);
+const pdfBytes = await toPdf(note);
+```
+
+The default font (Helvetica) only supports Latin text. Pass `fontBytes` with a Unicode TTF/OTF to support other scripts:
+
+```ts
+const pdfBytes = await toPdf(note, { fontBytes: await fs.readFile('NotoSans-Regular.ttf') });
+```
+
 ## Developer Notes
 
 ### Test Individual Suite

package.json

@@ -10,7 +10,7 @@
   ],
   "scripts": {
     "build": "tsc",
-    "clean": "rm -f tests/output/*.png tests/output/*.jpg tests/output/*.cpuprofile",
+    "clean": "rm -f tests/output/*.png tests/output/*.jpg tests/output/*.pdf tests/output/*.cpuprofile",
     "lint": "eslint .",
     "test": "vitest run",
     "test-watch": "vitest",
@@ -43,13 +43,16 @@
     "@vitest/coverage-v8": "4.1.10",
     "eslint": "10.8.0",
     "globals": "17.7.0",
+    "pdf-parse": "^2.4.5",
     "prettier": "3.9.6",
     "typescript": "6.0.3",
     "vitest": "4.1.10"
   },
   "dependencies": {
+    "@pdf-lib/fontkit": "^1.1.1",
     "color": "^5.0.3",
     "fs-extra": "^11.4.0",
-    "image-js": "^1.7.0"
+    "image-js": "^1.7.0",
+    "pdf-lib": "^1.17.1"
   }
 }