If you change the font-size in your custom PDF stylesheets, when you export a page to PDF, it does not have any effect in the generated document. It must get overridden later as adding the attribute !important make it work.
For example, this CSS does not have any effect in the generated PDF file.
To make it work, you need ot add the important attribute, as the example bellow:
This also happens for other parameters (i.e.: text-decoration).
Custom CSS should not be overridden by default ones.