I have a pdf that I can't copy text from.
I have tried several pdf readers: Foxit, Sumatra, PDF-XChange, mupdf
NONE of them allows me to simply copy the text!
What could cause this?
Some PDFs are protected so you can't copy the text. This link shows how to do it in Acrobat.
Another possibility is that your "text" file is actually an image, i.e. it is created from a scan or a screendump. In that case it is still readable, but there is no text in it. If this is the case, a single click on the "text" in Acrobat will shade the whole text in a blue box.
You can copy text with Foxit, are you sure that the PDF wansn't made from an image?
Yes, you can copy text, I am not allowed to post images but believe me, I've just downloaded and installed FoxitReader 5.3.1.0606, I saved a PPT presentation as PDF, opened it in Foxit, selected text and copied it