Some PDFs are scanned at horrendous quality yet seem to overlay this over actual text. I'm not exactly sure how this happens but I suppose an example is worth a thousand words. See this pdf:
Notice the terrible quality. However, if you select the text (tested in "Document Viewer 2.30.3", which is evince I think) by dragging the mouse over it or just Ctrl-A'ing it, suddenly the text is all clear. How do I remove the terrible quality overlay and just see the text?