Because i didn't find any other way to merge the files, i studied the text-based program i mentioned in previous post. It was surprisingly easy to use. I managed to join 3 files to one file.
Here's the instructions:
Merge PDF-filesYou will need these programs:
GSview 4.8 and
AFPL Ghostscript 8.53.
You may also need PS-driver from Adobe:
PS-driverIf you have any problem
Click here for help.
Also if you have problems, dont' hesitate to ask, i may be able to help.