Why a scan has no editable text
A scanner is a camera. It records how light or dark each point on the page is and stores the result as an image. It does not read the page, so it has no idea that a particular arrangement of dark pixels is the letter A.
So a scanned PDF is a stack of photographs in a PDF wrapper. Your eyes reconstruct words from the pixels; the file contains no characters, no fonts and no text layer for an editor to work with. The quick test is to try selecting text in any viewer — if your cursor will not highlight anything, it is a scan.
What you can do on a scan right now
Rather a lot, as long as you are adding rather than rewriting.
- 1Add text boxes anywhere — fill in blanks, correct a value by typing next to it, date the page.
- 2Sign it — type, draw or upload a signature and place it on the signature line.
- 3Highlight, comment and add sticky notes for review.
- 4Draw, and add shapes and arrows to circle or point at part of the page.
- 5Cover something with a filled shape, then type the replacement on top — the practical way to change a value on a scan.
- 6Rotate, reorder, delete and extract pages, and compress the file, none of which needs a text layer.
Changing the words themselves needs OCR
Optical character recognition analyses the image, recognises the shapes as characters, and produces a text layer that sits behind the picture of the page. Once a scan has been through OCR, its text can be selected, searched and edited.
Two things worth knowing before you rely on it. OCR is a best guess, and it makes mistakes — particularly with poor scans, unusual fonts, handwriting, tables and numbers, where a misread digit in an invoice total is worse than no text at all. And OCR gives you the words, not the design: an edited OCR document rarely matches the original scan's layout exactly.
This editor does not perform OCR. If your scan is good quality and the wording must change, run it through an OCR tool first, then edit the result here.
Getting a scan worth editing
If you are producing the scan yourself, the quality of the capture decides everything downstream. Scan at 300 DPI or higher, keep the page flat and square to the glass, and light it evenly — shadows across a page are the main cause of both unreadable scans and bad OCR.
If you are photographing with a phone instead, fill the frame with the page, shoot straight down rather than at an angle, and check the photo before you leave the desk. A grayscale or black-and-white scan of a text document is both clearer and much smaller than a colour one.