Can't highlight some PDFs?

[franmatt80]

I have a range of different PDFs downloaded from academic journals, and a few which I created by scanning book pages and saving as PDFs. I use two apps for highlighting sections of text on my iPad, and MacBook - Documents by Readdle on iPad and Papers on iPad an MacBook.

I keep coming across PDFs which won't allow me to highlight lines of text and aren't searchable ie can't do a search for words within a document.

Why is this, and is there anything I can do to make these PDFs searchable and able to highlight?

Thanks for your advice!

 

[MAH11]

I keep coming across PDFs which won't allow me to highlight lines of text and aren't searchable ie can't do a search for words within a document.

Why is this, and is there anything I can do to make these PDFs searchable and able to highlight?

I run into two common problems. First, if the scanned image has not been converted to text (e.g., via OCR), you can neither search nor highlight. If you have Adobe Acrobat, the solution is simple: run OCR.

The second problem is that many journals put text in a second "overlayer" (this may not be the correct term). I see this a lot in papers that I get to review. I can highlight useless information in the overlay, like the line numbers, but the actual text in the manuscript cannot be highlighted. Acrobat has the ability to "flatten" a pdf, which solves this problem (at least some time).

I have also had a problem in which the text looks fine on screen, but any attempt to copy text or perform a search yields nonsense. This only happens with pdfs from one specific funding program. I just suck it up every year and have never understood the root of the issue.

 

[campyguy]

Besides flattening/rasterization, this:
https://helpx.adobe.com/acrobat/how-to/security-permissions-protect-pdf-files.html - I use it often...