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Might be, but chances are that at some point in time you said yes to something on your phone that you didn't intend to. That deal where sometimes you try to open a file - and being helpful - you phone will give you several choices for "what app" you want to use to open a file with... AND then your phone
helps you even more with "always open" this type of file with this app.
It will drive you absolutely insane!
There are ways to pick the default application. That can be an aggravating thing but still possible.
I think the easiest FIX for your particular problem - trying to open a PDF - would be installing a new version of the free Adobe Acrobat Reader. The installation will set PDFs to open with Acrobat (which is what you need). Costs nothing to download. Definitely worth trying and if you want a little more help let me know what kind of phone you have an maybe we talk in person. Maybe a PM so your phone number isn't hanging out there in public...
EDIT: Seems like you found a solution. Good for you! Yep, sometimes bringing in the "Big Guns" (the ten year olds) are required.