Make Scanned PDFs Searchable: An OCR Best‑Practice Guide (Accuracy & Size)

Many PDFs are actually images — for example, phone photos of paper documents, scans of printed pages, or PDFs composed from images. Text in such files can’t be selected, searched, or copied. You need OCR (Optical Character Recognition) to recognize characters in the image and convert them into real, searchable text.

Do you really need OCR?

Open the PDF in your browser/reader and try selecting text: if you can highlight individual words, it’s a “text PDF”. If selection happens in blocks or not at all, it’s likely an “image PDF/scan”.
If text stays razor‑sharp when zoomed but can’t be edited, it may be vector shapes instead of real text. You can still run OCR to make it editable/searchable.

One‑click start: Online OCR

For the simplest approach, use:

OCR (Make PDF Searchable)

Which output should I choose?

Keep layout; only need search/copy: choose “Searchable PDF” (text layer over original page image).
Need to edit deeply: PDF to Word or PDF to Text.

Key steps to improve OCR accuracy

1) Preprocessing: orientation, order, and noise

Before recognizing, clean up pages to significantly boost accuracy:

Orientation/order: Organize PDF Pages to batch‑rotate sideways pages, reorder with drag‑and‑drop, and delete blank/advert pages.
Black & White/Grayscale (great for monochrome text docs): Black & White / Grayscale improves contrast and suppresses color noise, aiding OCR and later compression.
Rasterize (when complex vector/CAD content confuses OCR): Rasterize Vector PDF converts complex vectors to bitmaps to reduce recognition interference.

Resolution & clarity

Recommended resolution: for text‑heavy documents, ~300 DPI is enough; for small fonts or poor print quality, increase to 400–600 DPI.
Avoid over‑compression/blurriness: too much noise or blur can lead to misrecognition.

2) Languages and layout

Always match OCR language(s) to the document (Chinese/English/Japanese/Korean/Traditional Chinese, etc.). For mixed content, select all relevant languages.
Complex layouts (multi‑column, tables, footnotes, vertical text) can lower accuracy; consider zoning the page and recognizing separately, or exporting to Word for manual touch‑ups.

3) Choose the right output format

Searchable PDF: best for archiving/search/annotations; looks identical to the original but becomes searchable/copyable.
Word: best for deep editing, but complex layouts may require manual fixing.
Plain text: lightest format; easiest for further processing but without layout.

Typical workflows

Text scans (contracts/handouts/reports)

Organize pages: Organize Pages → rotate/reorder/remove blanks.
Optional B&W/Grayscale for clarity: Black & White / Grayscale.
OCR: OCR (choose correct languages).
File too large? Then: Compress PDF.

Mixed text + images (color materials)

Fix orientation/order first; avoid aggressive B&W to preserve image detail.
Run OCR directly; if size is a concern, compress afterwards. Prefer “Strong/MRC” compression (friendlier for color docs).

CAD/vector content causing OCR issues

Rasterize: Rasterize PDF
Optionally convert to B&W for higher contrast
Run OCR again

FAQ

Q: Too many recognition mistakes?

A: Improve source clarity and contrast; verify language selection; try B&W/Grayscale to suppress noise; for multi‑column/tables, export to Word and proofread.

Q: Table recognition is poor?

A: For complex tables, try PDF to Excel to extract structured data, or fix tables manually after OCR.

Q: Output file is too large to send?

A: After OCR, use Compress PDF. For monochrome text scans, B&W first then compress — sizes usually drop significantly.

Q: Document contains sensitive data — is online OCR safe?

A: Prefer local processing or trusted services. If sharing, “export only necessary pages” or create a flattened copy via virtual print.

Q: PDF is restricted from editing/copying — how to OCR?

A: If you have legal permission, first Unlock PDF to remove permission restrictions, then run OCR.

Pro tips

Work in this order: “organize → OCR → compress” to avoid recognizing low‑quality pages.
For Chinese/English mixed text, enable both languages to improve accuracy.
When orientations are messy across many pages, batch‑rotate first; correct order helps later search/sectioning.
For “multi‑source merges”, use Organize Pages to unify order before OCR; combine with Black & White and Compression to balance clarity and size.