Hello and welcome,
difficult decision you have to make
The 9" devices are ok for PDFs in natural size, e.g. for pages with formulas and embedded graphics. If it's only text a 6" reader will be enough because of the reflow function.
Dowside of the 9" is they are not very good for extended travelling because the screen is quite sensitive to breaking and they are a lot heavier than the 6", bad for everyday casual use and one-hand reading.
The 612 is a further development of the Pro series, quite similar to the 603 only without 3G support and with a WACOM touch display that can only be used with the right kind of stylus, but I've never seen one in real life and I don't know if they are still produced.
I'd go for the 622, it's available in MediaMarkt nearly everywhere in Germany, costs around 170 Euros and has the best PDF support for small devices that I have seen so far. Additional it has a standard capacitive touchscreen that you can use either with fingers or with every cheap iPad stylus for 99 cents
If you have time enough a very good idea might be to convert your text PDF books into *.mobi or *.epub format, that is a lot easier to use than PDF.
A very good tool for this is the free
MobiPocket Creator, the resulting *. mobi files can be converted to *.epub using
Calibre