For 2019.3, we added another PDF.
VRED Professional 2016 had been the last version of VRED that supplied a PDF versions of the documentation. All subsequent versions can be found online, but not in PDF format.
VRED Professional 2019.3 Documentation
VRED Professional 2016 Documentation
VRED Professional 2015 What's New
VRED Professional 2015 Simple UI
VRED Professional 2015 Hands-On