One of the basic premises of the Paperflite platform is that it allows you maximum maneuverability. We'd like to enable you to do as you like! That's why we constantly build features with 'you' in mind. :)
Now, if you want to change your document thumbnail, here's how you can do it easily:
Log in to Paperflite and click on the asset whose thumbnail you'd like to change. Notice the 3 dots in the top-right and click the option to edit.
Click on the pen icon in the thumbnail preview.
Clicking on it allows you to use a picture from your local computer, Pexels, Paperflite, and pick from pages. Once you're done, hit 'Update' at the bottom to save your changes.
You can follow the same process to edit the thumbnail that you've previously uploaded too.