I was a mac enthuiast back in the days of the 128k, Fat Mac, etc, but for a variety of reasons I had to wipe out and convert to Windows based machines. Now I am the fond owner of an iPad, which has become my number one go-to device. But I do have a problem because my desktop is a Toshiba (actually a laptop, but with large monitor, etc.). I use Good Reader a lot for reading very large pdf files, often around a thousand pages. This makes turning pages and other operations pretty slow. The help facility on Good Reader says that this can be sped up a lot if I first open a document in a Mac and then save it and transfer it to the iPad. Long story, now the brief question:I am thinking of buying a used Mac just for this job. Can anyone tell me how new an OS I need to run? I am assuming that the older it is, the less expensive it will be. Any recommendations? Thanks.