This is a little tough for me to understand. (I fully understand that you likely cannot write English perfectly since you appear to be Québécois, and I fully respect that. No hard feelings; please do not take offense.)
On the one hand, your post's title appears to indicate that you want to set the "Print to file" option as the default print option. But on the other hand, your post body would indicate that you instead want the PDF Printer to be the default.
Lacking any other information, I'll assume you want to set the PDF printer as the default.
If you use the now-default Brisk menu with MATE, then open up the menu, search for "Print", and choose the application which says roughly "Printer Configuration" or "Printing". If you use the traditional menu, go to System -> Administration -> Print Settings.
In the window that pops up, select the "PDF Printer", right-click it, and in the menu that pops up, click "Set As Default".
You can close the Print Settings now.
Hopefully that should set the PDF Printer as the default printer. If it does not, show us a screengrab of a "Print" dialog which exhibits this behaviour, and preferably also mention what application's print dialog is pictured. By seeing the symptoms, hopefully we can work this problem out.