I'm using Insert->Page Break to do just that, but thick black lines are appearing on the actual printed copy wherever I indicated a break. Can this be turned off?
I can confirm that this also happens to me in the default stylesheet. Perhaps you should contact email@example.com to file a bug report. As a workaround I suggest using cells with "\f" (form feed). Evaluate an input cell with it and copy the resulting output cell anywhere you want to have a page break.
If you wish to suppress the black line you can change this in the Style Sheet. For a particular Notebook choose Format > Edit Stylesheet... and paste this into the window:
Choose Yes if asked "Do you want to interpret the text?". Nothing will appear because the style is invisible (that's the idea after all) but it will have effect. Close the Style Sheet.
Linebreaks in this Notebook will now be invisible in the "Printout" environment (the default for printing and PDF export).
If you want to apply this to all new Notebooks you can create a custom Style Sheet and then change the global