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!