Preview fails to copy text

When trying to copy and paste text from a PDF in preview to a document in another program, text highlights but fails to actually copy, resulting in it failing to past because there's nothing to paste. I've already quit and restarted Preview and rebooted the computer, and the PDF is supposedly writeable. I was briefly able to copy from a copy of the PDF, but then it stopped working again. I know zilch about computer science, so if you know how to fix this, please try to explain without using too much jargon. Thanks!