For debugging purposes it's a good idea to keep .pdb files also for release version of your libraries. Unfortunately in default configuration Qt library doesn't generate .pdb files. It's a not big deal, create .pdb files for release version it's pretty simple.
1) Add compilation flags to .pro file
The easiest way how to create pdb files is pass /Zi switch to compiler and /Debug to linker.
2) Solution via mkspec
As pointed out in this QtProject article, another way is to edit mkspec so any future compilations will have this option turned on. It's necessary to modify correct mkspec file, for example
and add following lines (it should be sufficient to add only /Zi and /Debug switch):