That's often caused by the font specified in the PDF not being available on the platform where the PDF viewer is running, so a different font has been used instead.
Hmm. I may have been unclear--the PDF reads fine, but if I copy and paste some text into a text editor, I get the messed-up spaces. It seems as if the PDF doesn't encode text as text but just as a series of characters and locations, leaving spaces unrecorded, so when copy-pasting the reader has to guess from the distance between letters.