I want to amend some of my last year’s assignment and exam papers so I can use them again this year after making updates. Any suggestions how to update pre-printed material?
It sounds like you want to be able to scan a document, then edit that document.
Which is exactly what you should do. Many document scanning applications have optical character recognition (OCR) at the time of scanning. This means when you scan the document, the scanning application will recognize it as text, and create the document as an editable text document.
If this is not the case, and it scans as an uneditable pdf, you will probably need to apply some other program to that pdf - one that has OCR. If you do not have an application that will do this,
I recommend using Google Docs to upload pdfs and convert to text using Google's OCR.
I used to do this exclusively at a library job, digitizing old'ish documents, books, pictures, postcards, etc. We used ABBYY FineReader and it worked great. I doubt it's free, but there are open source alternatives.