What recognition can and cannot do
A scanned PDF contains pictures of words, not words. Nothing can select them, search them or copy them, because as far as the file is concerned each page is a photograph. Recognition looks at those pictures and works out which letters they show, then writes that text back into the document underneath the image.
It is a reading of your page rather than a copy of it. Clean printed text on a straight scan is recognised very accurately; handwriting, faint photocopies, tight columns and unusual typefaces are where mistakes appear. Always read the output before relying on it.
- The page still looks the same
- The original image stays on top. The recognised text sits behind it, so the document reads exactly as before but is now searchable.
- Language matters
- Recognition uses a model per language. Choosing the wrong one produces confident nonsense rather than an error.
- Quality in, quality out
- A 300 DPI scan recognises far better than a photograph of a page taken at an angle.