Can’t open PDF portfolio on iPhone

Sometimes when i try to open a pdf on my iPhone, I get this message instead "For the best experience, open this PDF portfolio in Acrobat X or Adobe Reader X, or later." and a button that says "Get Adobe Reader now!" 
The button opens iTunes, where I can click open to the app. But I can never open the pdf.

why is this? 
Please help it is really frustrating.