I have a Nokia Lumia 620 and I had connected it my computer it then showed the phone's C drive with 1.3 GB free. I thought let me delete some unnecessary files. it then shows folders like "Documents", "Pictures", "Videos" etc, when I opened Documents folder I cannot see any files, and it says 0 files, whereas I have some pdfs downloaded through Adobe Acrobat on my phone, but I am not able to access it or move it my computer, Can someone please help me out?
I would think the Documents folder is only for Microsoft Office files that you create locally on your phone (not on Onedrive). Anything that is opened in Adobe on your phone is probably stored in that app's local memory which you cannot access by connecting to your computer.
The easiest solution would be to try and email the PDF from the Adobe app on your phone and get it that way.
What you saw was not equivalent to a computer drive. You cannot access all of the files of the phone from here. What you saw was the shared files of the phone. All apps have their own isolated storage, or can also access this shared area. If it does, it will list that as a capability before you download the app. Most apps such as the PDF reader you mentioned save to their own private storage.