I sounds like the PRV is unfloated and maximized. Clicking the Restore icon in the upper right corner of the window below the main program's minimize/restore/maximize buttons should make it smaller so you can click the title bar icon at the upper left and choose Dock in Multidock.
 
 

Bingo, that's worked (clicking the Restore)! Thanks a lot brundlefly and all you guys.
I imagined this must have been a one click thing but could not find it myself...