there's quite a lot of info on the forum about this if you do a search. but any way...
It used to be possible to publish to EPS - what version of Archicad are you using, I think EPS died with plotmaker, so if you're on version 10 or 11 that's no longer an option.
I've found the best way is to save to PDF, I've never had any pixelation problems - what software are you opening the PDF in? If Photoshop, then you will be asked what resolution you want it to be, so you have the control over how good the image is. If possible open in Illustrator, and the PDF is converted to scalable vectors, so definitely no pixelation.
How are you saving to PDF? What settings?
Illustrator also opens DWGs so you could try publishing to DWG if PDF isn't working for you.