It would be simpler to answer this question if you could provide some answers to the following questions:
What application did you use to open the document? Browser plugin, Preview, Adobe Reader?
What e-mail program do you use? Have you tried to download the file locally to your home folder?
What are the permissions off the PDF file when on your disk (ls -l /path/to/pdf or right-click > Get info)?
Is the PDF password protected?
Could also be that the PDF link your boss sent you links to ftp or web server that only users with certain permissions can access. Maybe you need to specify a username and password to access the page?
there are lots of programs to open pdf file that everyone can use. For example: Adobe Reader. If this program give the error message too, try to upload your file to pdf recovery service: https://onlinefile.repair/en/pdf.html and than re-open it.