I would say that your PDF converter, or the Sony reader prs-650 is not capable of any more then that 171 mark. I was looking at the specification for the 650 and it says it can hold about 50,000 books at one time.
Try changing the magnification level for the pages. This has helped other people with the same problem. If the does not work try to convert to ePub next and this may be easier for the 650 to handle. Sometimes conversions can cause corrupt pdf files.
Finally, try to use calibre to transfer from PC to your hand held device.
http://yourpdfconverter.com