For some pdf doc's I downloaded from HP, if I copy some characters from the text and then paste them into the Find box, the characters show up as hieroglyphs (mostly squares) in the Find box. And then actually executing the Find using those characters doesn't work, because it can't find the very word that I copied them from! On top of that, if I manually type a word into the Find box (showing normal letters), it won't find it even though I can see it within the text on the page showing on the screen. Very strange! In documents other than these old HP documents, the Find function works fine.
I looked into the document properties (menu: File >> Properties >> Fonts) and see that there are 9 fonts that have names I have never heard of before, and those fonts say "(Embedded Subset)" after the font name and "Encoding: Custom" in the font descriptions. There were a few normal fonts listed before these for that document as well.