Many PDFs of old papers are only an image of text. Other papers have been converted with optical character recognition (OCR). By OCR'ed, I mean the text is electronically typeset, and so there are no artifacts from the scanner and good support for zooming in, copying-and-pasting, etc. In a properly OCR'ed document, the text layer is on top of the image level, so the reader can read the text comfortably; otherwise, the text will look ugly, as if the document has not been OCR'ed at all.
I find documents of the latter group to be very annoying. Is there any free service to fix such documents? Or other options to make reading such documents easier?
Example link: https://link.springer.com/article/10.1007%252FBF00117714