Hi Marco,
Printing the PDF document in Fit size is the default behavior of Syncfusion PdfViewerControl in UWP platform. We can see the same behavior in the printed output from Microsoft Reader also.
We have attached the document we have used, printed outputs taken from Microsoft reader and Syncfusion PdfViewerControl in the below link for your reference.
http://www.syncfusion.com/downloads/support/forum/124614/ze/output-1857892600.zip
Please let us know if you have any concerns.
Regards,
Praveena P