If you've ever come across a locked PDF file (ie, one that is completely read-only), you know how frustrating it can be. Locked PDFs can keep you from inserting comments, highlighting important info, or just copy and pasting information. When this happens to me, I use a couple of PDF unlocking websites to make my files easy to work with. Find out what they are when you read more.
Visit either FreeMyPDF or PDFUnlock and upload your PDF. If the site is able to unlock it, you'll be prompted to save the new file, which you'll now be able to modify. These services don't open any files that need a password to view, and the FreeMyPDF site stresses that the service should only be used for valid and legal purposes, so use your judgment about which PDF files you should unlock.