Document upload to Linkedin is not yet supported by the mobile app. but you can do it with chrome browser or any other browser supporting desktop mode, see the steps below.
- Open Linkedin.com in chrome browser.
- From chrome menu enable “Desktop Site” more.
- Top left corner of screen click on your profile image to go to your profile.
- Once in your profile find the edit button and tap there.
- Scroll below and find Media, tap on upload there to browse your resume.
- Give Title & Description to your resume.
And you are done.