If you're the one who are privileged to view it the sender should have sent you a password, too. Maybe in a separate post but nonetheless.
If it doesn't ask you for password, it isn't encrypted. Instead, it either may be written with a program your computer doesn't have what it takes to read. I've come across this issue with Open/Libre Office files in a computer that either doesn't have Microsoft Office or the version of it is too old. For some reason many file unsupported file formats try to open in a PDF reader, either Adobe or Foxit.
So, is it actually a PDF file or is it only trying to open in your PDF reader? If it has the extension of .pdf, it may also be corrupted. If it has but it's not corrupted, it may contain elements that only will open in Adobe Reader.
Summary: Check the extension! If it's not .pdf, find the appropriate program to open it. If it is .pdf, download and try to read it with Adobe Reader.
