How exactly are you opening the files?
Navigating to the network drive and double-clicking. Or using the Open Remote.
Is your solution hosted on a separate machine, or are you hosting the file and have the other person remote connect to it?
The main computer opens the .fp7 file directly on their machine. The other computer (which is on the same network) opens the actual FMP application first. Then uses the Open Remote to find that local host and file to open.
Does this answer your question(s)?
It sounds like you are hosting the file on a machine that is also being used for normal, day-to-day work tasks. It is possible that is part of the problem. If there is a heavy load on the machine already, just from whatever apps the user is running...adding to that hosting a FileMaker file could be compounding the problem. Test and see if it is happening when the user has no other apps running.
When you say "normal functions", are you just selecting the functions from the menu or are they part of a script?
How many records are in the file? How many records are you trying to save to PDF, when it freezes?
Most of the time I run into that problem, it is because the host machine is running out of memory (RAM) trying to perform a task.
OSX Version? Mac Model? RAM installed?
Monthly maintenance done? PDFs created how?
Open Activity Monitor (hd > applications > utilities) and watch the free or green Memory in the pie chart.
If it goes to no green when the FMP quitting occurs more RAM may help.
Open Console and check error logs, this may or may not help (hd > applications > utilities)
If not Snow Leopard, AppleJack run monthly may help.
If it only occurs during a print or PDF creation, you may have corrupt font caches or corrupt fonts.