I had some strange issues copying text out of a PDF on my iPad (I always got a bunch of ????), so I decided to run my pdf through xpdf to see if that would fix things:
pdftops naughty.pdf intermediate.ps
pstopdf -o good.pdf intermediate.ps
After the first command ran though, I got the following error:
Config Error: No display font for 'Helvetica-Bold'
Config Error: No display font for 'Times-Bold'
Config Error: No display font for 'Times-BoldItalic'
Config Error: No display font for 'Times-Italic'
Config Error: No display font for 'Times-Roman'
On a Linux installation, I'd typically resolve this by installing a font package, but I'm not sure what to do on OS X.