Hi, just want to say thanks. Have resolved this issue now.
For me I fixed the issue by first going to IIS (on the server), go to the Site > Authentication >Anonymous Authentication - Edit this and change to Application pool identity. Ok and restart. [After doing this I noticed in php errors it was giving the mpdf un able to write to windows\temp folder]
I then checked the security and users for the wwwroot directory and noticed the extra xx_IUSRS group (xx is our name so maybe different on yours) so added this to the windows\temp with Full control.
Tried printing on OSTicket and voila worked!