Why can't you answer my question?
"What is PDF?"
Actually the full answer means reading ISO 32000-1:2008 (the ISO standard for PDF).
Myself, I'm fond of ISO 32000-1:2008 but then I'm a youngster of 60 something 8^)
A good alternative is at:
"How do I find 'manage add-ons in adobe?"
Really no answer to that as "adobe" is the company.
Regardless, "manage add-ons" is something that is generally related to web browser applications.
(Example: MS Internet Explorer; Chrome; Safari; Fire Fox, etc.)
Because you speak of online activity I suspect your issue(s) reside with the browser.
For "manage add-ons" read the user Help for the browser you are using.
Some related information available from Adobe:
"Can't view PDF on the web"
"Display PDF in browser"
You certainly asked a very big question with a 1,000 page answer.
And, if that happens not to help please let us know the following (please answer all three questions, or tell us if they don't make sense).
* what system do you have. For example, answer Windows 7 or Mac OS Mavericks.
* what browser do you use. For example, answer Internet Explorer or Chrome.
* what error message do you get. Please give us the EXACT words even if they seem to make no sense.