Adobe Digital Editions book shows only a narrow column on my monitor

I just purchased an e-book requiring Adobe Digital Editions (the new DSM-5 TR).   The authorization process went ok.  The book pages appear as one or two narrow columns on my monitor, each occupying about 15% of the width of my screen.  At a comfortable text size, this means that each column is about 6 words across.  Most of my large monitor screen is blank (just a mass of white).  This is not affected by changing magnification or text size.  The PDF view menu is greyed out, so it's not letting me choose to "fit width" etc.  

I would like to be able to read this e-book as I would any other PDF or Kindle book etc. , by having each page occupy 50-100% of the width of the screen.  

Please let me know if this can be fixed!

Thanks a lot!