OCR looks at a picture of a page and produces text. On a clean 300 DPI scan of ordinary printed text, accuracy is very high. On a phone photo taken at an angle in poor light, it is not.
The single biggest lever is scan quality, and it is decided before OCR runs. Rescanning at 300 DPI in greyscale beats any amount of software cleverness applied afterwards.
A scanned PDF contains no text. It contains pictures of pages, and the fact that those pictures show words is meaningful to you and to nothing else. Search returns nothing, copying is impossible, and a screen reader finds an empty document.
OCR is what closes that gap, and understanding roughly how it works explains why it is so good on some documents and so poor on others. The guide to why a scanned PDF is not searchable looks at the same gap from the file's side.
What the software does
Recognition runs in stages, and each one can go wrong in its own way.
Layout analysis
Before reading anything, the engine works out the structure of the page: which regions are text, which are images, where the columns are and what order they should be read in. Mistakes here scramble the output even when every individual letter is read correctly.
Line and character segmentation
Text regions are split into lines, then into individual characters. Touching letters in a low resolution scan get merged into one shape, and broken letters in a faded one get split into two.
Classification
Each shape is compared against the engine's model of what letters look like and assigned the most likely candidate, with a confidence score.
Language correction
A dictionary and language model fix ambiguous cases. This is what turns rnodern into modern, and it is why choosing the right language matters so much.
rn from m, or l from 1, from shape alone. The language model resolves those cases by asking which reading produces a real word. Point it at the wrong language and that correction stage works against you, turning plausible text into confident nonsense. PDFslime's OCR ships only the English model for now, so text in other languages loses this correction stage and needs a closer proofread.The four things that decide accuracy
Almost all of these are properties of the scan, not of the software.
- Resolution300 DPI is the target
- ContrastSharp black on white
- SkewStraight, not leaning
- TypefacePlain serif or sans
- Low resolutionLetters merge together
- HandwritingLargely unreadable
Resolution is the one to get right. Tesseract's documentation says the engine works best on images of at least 300 dpi. Below that, the strokes of a small letter are only a few pixels wide and neighbouring characters begin to merge into one shape.
Enough detail to separate touching letters without inflating processing time.
Strokes become too thin to tell apart. Rescanning beats post-processing.
A colour scan is bigger and reads no better. Keep colour only when the image itself needs it.
Two kinds of output
OCR can produce two quite different things, and choosing the wrong one is a common source of disappointment. PDFslime's OCR tool writes the searchable PDF and can also hand you the recognised text as a .txt file from the same run. It boxes every word it read on the page and marks the uncertain ones in red, which is the quickest way to see where the text layer is weak.
The page, plus hidden text
- Looks exactly like the original scan
- Invisible text layer sits behind the image
- Search and copy work
- Right for archiving and filing
Just the words
- Much smaller file
- Every recognition error is visible
- Formatting, columns and tables are lost
- Right when you need to edit or reuse the words
What OCR still cannot do
Expectations have been raised by how well it handles clean printed text, and there are categories where it remains weak.
Handwriting is the main one. Cursive handwriting is effectively unreadable to standard OCR, and even neat printing gives poor results. Complex tables are another: the engine may read every cell correctly and still lose the structure that made the table meaningful. Multi-column layouts can be read in the wrong order if layout analysis misjudges the columns. And very stylised display typefaces, which differ substantially from the shapes the model learned, produce errors that the language model then confidently corrects into the wrong words.
Make a scan searchable in your browser
Recognition runs locally. The document is never uploaded and never leaves your machine.
Common questions
Getting a usable result.
01What does OCR do to my PDF?
It examines the picture of each page, works out where the text regions are, identifies each character shape, and writes the result as an invisible text layer positioned behind the original image. The page looks unchanged and becomes searchable.
02What scan resolution should I use?
300 DPI when you scan, which is the resolution Tesseract's documentation names as its best case. If the PDF already exists, its resolution is fixed. PDFslime's OCR redraws each page at 150, 200 or 300 dpi before reading it, but a higher setting cannot add detail the scanner never captured.
03Does scanning in colour improve accuracy?
No. Tesseract, the engine PDFslime's OCR runs, turns each page into black and white before it reads it, so the colour never reaches the recogniser. Scan in colour only when the archived image needs it, such as a form with coloured stamps.
04Why is my OCR output full of errors?
Usually the scan rather than the software. Low resolution, poor contrast, a skewed page or an unusual typeface all degrade recognition, and no amount of reprocessing recovers detail that was never captured. Rescanning at 300 DPI in greyscale, straight, fixes most cases.
05Can OCR read handwriting?
Standard OCR effectively cannot. Cursive is unreadable to it and even careful printing gives poor results. Specialised handwriting recognition exists and is a different technology with its own limits.
06Should I choose searchable PDF or text only?
Searchable PDF if you want to keep the document looking exactly as it does and simply be able to find things in it. Text only if you need the words themselves to edit or reuse, accepting that layout, columns and images are discarded.