Why do some PDF documents download and some don't

[Android Central Question]

I can open some PDF documents on my phone and some I can't open at all.

 

[ManiacJoe]

Which phone?
Which version of android?
Which carrier?

We need to be careful with the terminology. "Download" and "open" are not the same thing.

Does your phone have a PDF viewer installed, for example Adobe's Acrobat Reader?
Some email apps have PDF reading ability built into them.
Which apps are you using when you have problems?