I get scanned book pages from time to time as PDF to read
But when I try to mark them on the laptop, of course, it doesn't work, because it is theoretically just the image of a text
Are there any programs I can use to mark texts in a meaningful way?
Or can I turn the scanned text back into 'correct' text?
There are text recognition programs or apps that use them to create a text document. Not working 100%, text still needs to be reworked. Otherwise, a colored color field would have to be placed around the area of the scanned pdf.
Search the Internet for OCR scanners (Optical Character Recognition) or take a look at Microsoft Office One Note, there you can extract the text from images as far as I know.
I myself have generally found that everything works only to a limited extent, especially if you have pages in a two-column layout, which is a problem
Are there any programs I can use to mark texts in a meaningful way?
Yes, annotation programs for PDF files, for example Xournal: http://xournal.sourceforge.net/
However, with larger PDF files (and documents scanned as images are usually large) it should be quite challenging for CPU and RAM, so it may feel a bit sluggish.
Or can I turn the scanned text back into 'correct' text?
There's also that is called OCR. However, the costs and are not too reliable, so you have to go through the generated text again and correct errors, not very convenient.
