are you using special fonts in that pdf? maybe the font is not availaibe on the machine
First make sure the program they are printing from is 64 bit and not 32 bit. Then make sure you are using the HP Jet Direct as the port. If the problem continues please consider contacting your local support centre for additional assistance.
When the one MAC user prints documents we get the following:
macOS Sierra 10.12.6
Using the latest driver and the HP Directjet Port.