Passwords can be assigned to Acrobat documents which prevent copying and similar actions.
They can be removed. If you know (or guess) the original password (yes, I know, but it's an option) change security to "None" in the "File -> Document Security" menu option.
There is software to remove passwords. PDFRemover seems good, but unless you pay the $30, then it only does half the document. I also found APDFPR - seems to install and work reasonably well (ah - only decrypts first 2 pages).
Other PDF password remover programs can be found by googling for them.