Why Can't I Search My Scanned PDFs? Text PDFs vs Image PDFs on Windows

2026-10-15·6 min read

Quick Answer: A scanned PDF is usually a stack of page images with no text layer, so there is nothing for a search tool to read — Windows Search, Acrobat's search and LocalSynapse all come back empty. To check, open the PDF and try to select a word or press Ctrl+F: if you can't, it is image-only. The fix is OCR (optical character recognition), which adds a hidden text layer — for example with the OCR feature in PDF editors such as Adobe Acrobat, or the command-line tool OCRmyPDF. After you replace the file with its OCR'd version, any content-search tool can find it. LocalSynapse does not run OCR itself.

You know the contract is in that folder. You search for the client's name and get nothing — even though you can see the name on the first page when you open the file. The problem usually isn't your search tool. It's that the PDF has no text in it at all.

Two kinds of PDF that look identical

On screen, a PDF page is a PDF page. Underneath, there are two very different kinds:

Text PDFs are produced by software — "Save as PDF" from Word, "Print to PDF", most invoices and statements generated by a system. The words are stored as text. Every search tool can read them.
Image PDFs come from scanners, phone scanning apps and fax-to-PDF services. Each page is a photograph of paper. There are no words in the file, only pixels that happen to look like words.

Search tools — Windows Search, Adobe Acrobat's search, and LocalSynapse alike — read the text that's stored in the file. An image PDF gives them nothing to read. That's why the same folder can be half-searchable: the generated PDFs show up, the scanned ones don't.

The ten-second test

Open the PDF in any viewer — Edge, Acrobat Reader, whatever you have.
Try to select a single word with the mouse. If the cursor draws a box over the whole page instead of highlighting a word, it's an image.
Or press Ctrl+F and search for a word you can clearly see. No match means no text layer.

Some files are mixed: a generated cover page followed by scanned signature pages. Test a page in the middle, not just the first one.

The fix: OCR

OCR (optical character recognition) looks at the page images, recognises the letters, and writes an invisible text layer behind them. The page looks exactly the same, but now the words can be selected, copied and searched. Options on Windows:

PDF editors such as Adobe Acrobat offer an OCR feature for scanned documents. Check your edition's feature list.
OCRmyPDF is a command-line tool that adds a text layer to scanned PDFs. It's well suited to processing a whole folder; follow its documentation for installation on Windows.
Your scanner's own software often has a "searchable PDF" option. If you scan regularly, turning that on fixes the problem at the source for every future document.

Two practical notes. OCR quality depends on the scan — skewed, low-resolution or handwritten pages will come out with errors, so search for distinctive words rather than long exact phrases. And keep the originals until you've checked the OCR'd versions open correctly.

A different problem: PDFs with garbled text

Occasionally you'll meet the opposite case: the PDF does contain text, but when you copy a sentence and paste it, you get nonsense characters. The page displays correctly because the font draws the right shapes, but the file lacks the information that maps those font codes back to real letters.

Search tools that index that text end up indexing garbage — the file appears in results for random strings and never for the words you can see. LocalSynapse detects one common kind of this: when every page of a PDF comes out garbled, it marks the file as unreadable rather than putting nonsense in your index. Running OCR on such a file usually produces a usable text layer, because OCR reads the page as an image and ignores the broken font data.

What this means for LocalSynapse

To be plain about it, since it's the question this post exists to answer:

LocalSynapse does not run OCR. It reads the text layer a PDF already has. An image-only PDF is still found by file name, but not by its contents.
Once a file has been OCR'd, it's searchable like any other PDF — by keyword or by meaning, alongside your Word, Excel and PowerPoint files. When you replace the scanned file with the OCR'd one, LocalSynapse notices the change through Windows' file-change notifications and re-indexes it.
Encrypted PDFs that need a password to open can't be read, so their contents aren't indexed either.

Files and document contents never leave your PC. LocalSynapse sends anonymous product-usage counts and technical health metrics only. You can turn this off in the app.

If most of your "unsearchable" PDFs turn out to be text PDFs after the ten-second test, the cause is elsewhere — our guide to searching inside PDF files on Windows and the post on why Windows Search misses files cover the usual suspects.