Scanned PDFs contain only image data, so search finds no text because there is none. Run the file through OCR to add an invisible text layer over each page. The page looks the same, and search, copy and screen readers start working.
You open a scanned contract, hit Ctrl-F, type a word you know is on the page, and the reader says "no results". You can see the word right there. The reader claims it does not exist, and from where the reader stands, it is right.
A PDF is two documents in one
A PDF page can hold two separate things: the picture you see, and text the reader can search. A scan has only the first.
Real characters underneath
- Text is stored as actual characters, not pixels
- Search finds every occurrence
- Copy-paste produces real text
- Screen readers can read aloud
Just an image on the page
- Only pixels, no character data
- Search finds nothing
- Copy-paste grabs the whole page as one lump
- Screen readers have nothing to read
When you scan a paper document, your scanner takes a photograph of each page and wraps those photos in a PDF container. The reader treats each page as one big image. The "text" you see is pixels arranged to look like letters. There is no character data stored anywhere in the file.
The three-second test
Not sure if your PDF is a scan or real text? Try this before doing anything else.
Open the PDF in any reader
Adobe, Preview, Chrome, whatever you already have. It does not matter which one.
Click and drag on a single word
Pick a word in the middle of a paragraph, not a heading or logo.
Watch what gets selected
If the word highlights cleanly by itself, the PDF has real text. If the cursor grabs the whole page as one blue rectangle, the PDF is a scan.
What OCR does to the file
OCR stands for optical character recognition. It is the software equivalent of what your brain does when it reads a scanned page: look at the pixels, recognise letter shapes, decide what each character is.
Running a scan through OCR does not change what the page looks like. It adds text written in invisible ink over the image, each word placed where the recogniser found it. When you search or copy, the reader uses that layer, so a search hit highlights the printed word itself. When you look at the page, you see the original scan.
Tesseract's own guidance: recognition holds up at 300 DPI or more. Faint copies and faxes fall short at any resolution.
PDFslime's OCR loads Tesseract's English model only, so other languages come out garbled. Every page is read on your own processor, so a phone takes longer than a laptop.
The text layer is a few words per line of print, tiny next to the image data.
What affects OCR accuracy
OCR works best on a clean, straight, high-resolution scan of printed text. As the input drifts from that, accuracy drops in predictable ways.
- Resolution matters most. Tesseract's own guidance asks for at least 300 DPI. 200 DPI suits most office print; go lower and small fonts start to break up.
- Contrast comes next. Black on white works best, and faded receipts, coloured backgrounds and low-contrast originals produce more errors.
- Standard serif and sans-serif fonts are easiest. Handwriting, calligraphy and heavily stylised display fonts are much harder.
- A page scanned at a slight angle confuses line detection. PDFslime's OCR corrects a lean automatically; other tools may need the page straightened first.
- The engine has to know the language. See the multi-language note below for what PDFslime's tool reads today.
- Simple single-column pages read cleanly. Newspapers, magazines and forms with mixed columns produce reading-order surprises that need cleanup.
Running OCR without uploading the file
Most online OCR services want you to upload the file to their server. For anything sensitive (contracts, medical records, legal filings, tax documents), that is a real problem. The alternative is to run OCR in your browser.
Tesseract, the long-standing open-source OCR engine, has been compiled to WebAssembly, so recognition can run inside the browser tab itself. PDFslime's OCR PDF tool works this way. The English language data is served from the site and loads once, and after that nothing further is fetched. Each word it reads is boxed on the page, with the uncertain ones in red, so you can check names and figures before downloading the searchable PDF.
What you can do once OCR is done
Once the PDF has a text layer, everything that needs text starts working. If what you want is the text itself, in Word or a plain file, extracting text from a scanned PDF covers that next step.
- Ctrl-F finds every occurrence, not just the ones your eye caught.
- You can copy paragraphs into Word or Notes as real, editable text.
- Screen readers have words to read aloud.
- macOS Spotlight and Windows Search can index the content, so the file turns up in later searches.
- Document management systems, note apps and cloud storage can index it too.
- Translation apps and reading assistants that need text can work with it.
Edge cases and workarounds
Handwriting
Traditional OCR engines struggle with handwriting. If the file is mostly typed text with a handwritten signature or a few notes, run OCR normally and accept that those handwritten portions will not be captured. If the file is entirely handwritten (personal notes, historical documents), OCR results will be too rough to be useful without heavy cleanup.
Multi-language documents
Tesseract can combine language models, so a page with English text and a Spanish quote reads well with both selected, at some accuracy cost per added language. PDFslime's OCR tool ships the English model only for now. Other Latin-script languages still read reasonably with it, though accented characters may be simplified, and characters outside Western European alphabets are not written into the text layer at all.
Very old or damaged scans
Faded typewriter carbons, low-contrast microfilm captures, or damaged historical documents may need image preprocessing (contrast adjustment, deskew, despeckle) before OCR. PDFslime's tool straightens a lean itself. For a faded original, raising the contrast in an image editor first, or rescanning at 300 DPI if the paper is still to hand, helps more than anything done afterwards.
Add the text layer your scan is missing
Recognition runs in your browser. The scan is never sent anywhere.
Common questions
Details that come up when running OCR for the first time.
01How do I know if my PDF is a scan or real text?
Try to select a single word by clicking and dragging with your mouse. If the cursor highlights one word cleanly, the PDF has real text. If the cursor grabs the whole page as one image, the file is a scan without a text layer. You can also try Ctrl-F for a word you can see. No results means no text layer.
02What does OCR do to the file?
OCR reads the pixels on the page and recognises letters, then writes them into the page as invisible text placed over the image. The text is drawn in a mode the PDF specification defines as invisible, so the glyphs are positioned and sized but nothing is painted.
03How accurate is browser-based OCR?
On clean 300 DPI scans of printed text, it gets almost every word. It falls away on faded copies, faxes, handwriting and low-resolution captures. Check the words the tool marks in red, since names and figures are where a wrong character does the most harm.
04Will OCR change how the file looks?
No. The scan keeps its own pixels and only gains the invisible text. The one exception in PDFslime's tool is a sideways page, which it turns upright if you leave the rotation option on.
05Does OCR increase file size?
Only slightly, because a page of text is small next to the image of that page. Scans are large to begin with, though, so if size matters, compress after OCR; compressing first and then running OCR on a softer image costs accuracy.
06Does PDFslime upload the file for OCR?
No. Our OCR tool runs Tesseract via WebAssembly inside your browser tab. Files are read into local memory, processed locally, and never leave the device.