QPDFDocument renders pdf text with wrong character spacing

When rendering a PDF document extra spaces or wrong spacing is used between certain characters. From what I could see it doesn't happen with all the documents, only with specific ones.

The document looks ok when opened in another pdf viewer. Belowe you can see some examples. I tried all the rendering flags, I also tried changing the resolution, but nothing seems to work. 

The issue is visible also in the PDF Viewer example from QtCreator. 

I attached also an example of a pdf with which it reproduces.