A, I'm sorry but the only thing you can upload to dropbox (or any other online storage space) is a file. WikiTree does not produce a gedcom file for you to download - it only produces the gedcom content on a screen and it is then up to you to store it as a file. To do that, you have to be able to copy it and paste it into a text editor, from which you will be able to save it as a file.
I wish I could provide more help, but I don't know anything about Polaris Office or what tablet you are using. As you say, you should be able to paste it, but I am not able to give you instructions for how to do that. Hopefully, someone else will show up here who is familiar with that program.