Yes, you can download the online 12th marksheet in the form of a PDF file and then upload it to JEE Advance document upload window or you can scan your 12th class marksheet and then upload it. In both, the way you need to keep one thing in mind that the size of your PDF file is in the range mentioned on the site otherwise your file will not be accepted.

And upload all the documents before 5th June 2018. Don't worry about the documents too much as all the original documents will be verified again when you will be joining any of the IITs.

Feel free to ask if you have any more queries. All the best!!