How can I obtain PDF from Orcad .dbk?
The simplest solution may be to use a PDF virtual printer. This allows any Windows software with a "print" function to print to a PDF instead of paper. I've had good luck with Bullzip, but there are many others, including CutePDF, PDFCreator, PrimoPDF, pdf 995, and doPDF.

I've never used any of these specifically with Orcad, but I don't see any immediate reason why they shouldn't work. They definitely work well with Ultiboard, Multisim, MS Word, and a number of other applications.

Of course, this assumes you have a working copy of Orcad. If you don't, there appears to be a viewer available for download from Cadence here. I haven't tried it, unfortunately.