The LaserJet 2600n is a relatively cheap 'host-based' device. This means that it utilises the processing power and resources of the 'host' computer to process the print job, rather than this being done within the printer itself.
Such print devices do not understand PCL (5 or 6) or PostScript language print streams; the 'host-based' driver on the PC generates a simple raster image of each page, compresses this, and then encapsulates it in a very limited 'stripped-down' PCL5 or PCL6 (set of) commands (I can't remember which).
I believe HP calls this language JetReady (but they don't appear to publish any details). I think that the protocol also relies on a bi-directional connection (but, again, I'm not very sure of this).
As 'goinfotek' points out, this should not prevent PDF files from being printed; any limitation will be in the capabilities (or lack of) provided by the host-based driver.