Hi Nada -
I am not sure if you are trying to create a PDF from a CA Plex report, or if you mean PDF's in general. I will second using Crystal Reports, BIRT, Jaspar Reports as good options for as a replacement for Plex reports. CM WebClient for CA Plex has a Crystal Report integration where you can call a CR Report from CA Plex, passing parms, etc. and see the results in a browser view and then save to PDF, or also save the report to PDF directly.
If you are looking at general programmatic conversion of source docs to PDF, I would also look at the Lead Tools SDK or the Cute PDF SDK. I am aware of CA Plex folks using these SDK's from Plex C++ apps successfully. They should work from C# as well.