Discourse allows you to create a PDF of a topic by using the CTRL+P keyboard shortcut from within a topic. That command will open a new browser window that allows you to save the topic as a PDF:
The shortcut for creating a PDF is given in the Actions section of the keyboard help window.
I would really like to be able to modify the size of the printed text. How can I do that? I’ve been looking around on meta and can’t find the right documentation for this.
I think this depends on your operating system and the application that’s installed with your printer. On Ubuntu I can click the “More Settings” link that’s shown in the screenshot above, then set a custom text size from the settings.