Hi Geoff,
Actually, PixInsight includes a complete printing system. It is enabled by default on FreeBSD and Linux, but disabled by default on OS X and Windows. You can enable it by running the PixInsight core application with the "--with-printer" command-line argument (without quotes). This will enable the printing menu commands, namely FILE > Printer Setup, FILE > Printer Color Management, and FILE > Print.
Printing is now disabled by default on OS X and Windows because I want to improve it significantly to make use of native printer features on these platforms, especially on OS X. One of the areas where we need more improvements is color management when printing directly to physical printer devices.
So you can indeed print from PixInsight, including generation of PDF documents. Our printing system makes use of the advanced pixel interpolation algorithms and accurate color management transformations implemented on the PixInsight platform.