I just upgraded our company from Enterprise PDM 2015 SP3 to SolidWorks PDM Professional 2016 SP4 this past weekend. We discovered that users with Adobe Reader XI experienced PDF files would open in Reader XI full screen when simply selected (one click). This was only happening with PDF's in PDM, not their local drives. Upgrading these folks to Adobe Reader DC resolved the issue. Hopefully this helps others experiencing this because I couldn't find anything already published on this.