Can someone help me to understand that if i upload a file/document to Workfornt, does it stored in Workfront or not, how long it will be stored.

to my knowledge, files will be stored on WF servers until you delete them (than they land in recycle bin from which they can be restored within 30 days)  OR stop licensing the app. Than a 30 day retention policy kicks in. After that all data will be removed from WF servers. That's at least what I've learned from WF support some time back. 

What also might be relevant:

While there is no official limit to single file uploaded (it can be any size?), every WF instance has limited storage quota, which can be checked by system admin: Setup>System>Customer info

Hope this helps