If glyphs in a font are named according to Adobe's "uniXXXXYYYY...." Glyph Naming Convention then you can search in PDF files and cut and paste text from PDF to other applications since Acrobat can re-construct the Unicode characters of the original text by parsing the glyph names. This works even for ligatures which do not correspond to a single Unicode code point and have no Unicode index in the font.
If glyphs in a complex script font are named in any other manner this doesn't work - so results will differ from font to font.
I also don't know which non-Adobe PDF viewers support this.
- Chris