"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:
http://en.wikipedia.org/wiki/Portable_Document_Format
"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:
Browser Troubleshooting
"Can't view PDF on the web"
http://helpx.adobe.com/acrobat/kb/cant-view-pdf-web.html
"Display PDF in browser"
http://helpx.adobe.com/acrobat/using/display-pdf-browser-acrobat-xi.html
.
Be well...