Attached example.zip includes an example application (modified from $QTDIR\examples\webkit\preview example app) which can be used to reproduce the error. Steps:

• Launch the application and select File - Open. Open a file 'transparent_PNG_sample.html' which is also attached on the .zip.
• Select Print - Print PDF. Type file name and press Save.
• Open 'GIF_sample.html' by selecting File - Open.
• Select Print - Print PDF. Type file name and press Save.

Compare the PDF printing output for 'transparent_PNG_sample.html' and 'GIF_sample.html' (attached .zip file includes examples of printing outputs).

--> Error: PDF created from 'transparent_PNG_sample.html' includes text with bold font. PDF created from 'GIF_sample.html' includes text with non-bold font. The only difference between these html files is that 'transparent_PNG_sample.html' uses transparent PNG file instead of GIF file.

Tested with Qt4.7.4 and Qt4.8.0RC.