If one reads an html file to a QTextDocument and have also set a helvetica font for the html document, then the pdf created shows the characters incredibly small.
The attached application can be used to reproduce the issue.
It has been tested on Qt 4.8.2 which was configured with:
./configure -debug -prefix . -nomake examples -nomake demos -nomake docs -embedded -qvfb -confirm-license
When you run the application via qvfb, the characters in the pdf are small.
This doesn't reproduce on desktop Linux (tested with Ubuntu 10.04)
If one removes the helvetica from test.html, the characters are displaying fine: