Testcase:

<iframe style="display: none" src="test.pdf"></iframe>
<button onclick="frames[0].print()">Click me</button>

Load that over HTTP from somewhere that has a test.pdf. Wait a bit. Click the button. Instead of the PDF getting printed properly, you will get the "The PDF is not fully loaded for printing" alert.

I haven't managed to sort out the PDF viewer code enough so far to figure out why this is going on. Brendan, do you know offhand?

I put a live testcase at http://web.mit.edu/bzbarsky/www/testcases/pdf-printing/print-with-display-none.html