Have you checked the font list in your PDF (ctrl-D>fonts)? I would guess that either you do not have display smoothing turned on in the preferences, or your fonts are Type 3 bitmap fonts. Can you post a sample of the PDF for us to look at?
Thanks for the answer! It was the display smoothing.
Is there any option to embed this function into the pdf? Just to be sure people will get the best look of the document?
Or does it depend on the preferences on each pc?