I'm afraid the Sony PRST1 will not display epubs in any right-to-left language. Although the characters can be made to display with a custom font, they will display backwards, i.e left-to-right.
The PRST1 should be able to read an Arabic PDF, however you may not enjoy the experience on any of the 6" ereaders. Some PDFs are bearable and some are not because they have usually been designed to be read on paper, e.g. at A4 size. It depends on each particular PDF. You can often make them more bearable by using one of the free PDF cropping utilities for the PC to get rid of the useless white margins. The PRST1 does have a rudimentary cropping option but it's not wonderful.
On a slightly more positive note, if most of your PDFs are 2-column or 3-column then the PRST1 does have additional features for displaying them. It also has the ability to adjust contrast which can improve legibility.
Just for additional info, I recently bought an Android smartphone and was surprised to find that some of the Android apps are actually better for reading PDFs than any of my e-ink devices, even though the phone's screen is smaller. I'm sure that reading PDFs on a tablet, 7" or above, is even better.
I don't know anything about the Cybook but if you don't get an answer here then I would suggest you ask a specific question about Arabic in the Cybook forum where a current owner may be able to advise you.
P.S. Welcome to MobileRead
