PDF's can be printed directly.
Postscript files saved on the N900 can, also.
Aside from any files produced on the N900, pdf or postscript files can be received via email or retrieved via a network drive to the N900.

The lack of N900 printer connectivity has mainly made itself most conspicuous at work.

Otherwise, an absence of N900 apps that support printing can be explained by the fact that the N900 does not support printing.