There are basically two types of PDF protection - the original PDF creator can either restrict opening the PDF file itself or he can restrict others from modifying, printing or copying text and graphics from the PDF file. Here are a few possible workarounds:
When there are Copying or Printing Restrictions..
Say you want to print a couple of pages from the PDF document but the document settings won't let you do that.
Open the document in Acrobat Reader or Foxit and capture the PDF page as an image using any free screen capture software. If there are multiple pages, you may try SnagIt since it can autoscroll and capture multiple pages of the document in one-go.
If you want to copy just a portion of text from some PDF page, use a screen capture tool with OCR features (like Kleptomania, Capture Text discussed here)
Alternatively, you can invest in commercial solutions like Advanced PDF Password Recovery from ElcomSoft and PDF Password Remover from Very PDF.
(These utilities may not recover the password for you - they'll just remove the restrictions from the password protected file)
When there are Document Opening Restrictions..
This is a very tricky case and there's no straight-forward solution to read PDF documents that are password-protected at the Open level.
The software will use methods like Brute Force, Key Search and Dictionary Attack to guess the password. They will try to use all possible character combinations as the password and so the process might take hours or even days and would really depend on your computer's processing power.
Legal Issues: - you maybe surprised to learn that these PDF password cracking software are absolutely legal and Microsoft even awarded ElcomSoft a Gold Certified Partner status.