The PDF is probably password protected for a reason - somebody decided to disallow various things, including conversion to some other format. The only legal thing to do is to get in touch with the owner or creator of the PDF and ask them for either an unprotected version of the file, or maybe a Word file right away.
Thank you so much. I finally accomplished the task and made it work.
Hi Elsa, does the password protection prevent you from opening the file, or just from doing certain things like converting it to Word? In the case of the former, as Olaf points out you will need the password from the document owner. If the later, and if you are sure you have the legal and ethical right to do so, Google will easily find free services to remove the password protection. PDF to Word conversion is kinda hit and miss though, so please do not have too high of expectations.