Here's a few pointers from the FM help files. Perhaps one of these might help in your situation.
Choose Insert menu > PDF.••
NB: If you use the insert file option the PDF will not be interactive.
In some cases on a mac, removing the adobe internet plug-ins from Macintosh HD>Library>Internet Plug-ins ... may resolve this issue. In more recent examples of this issue in my own customers, reinstalling FileMaker Server 14v4 (using full installer file) after they updated to Mac OS X El Capitan, has resolved the issue.