HI @Elia ,
Could you please share a bit more about your scenario?
For your first question, do you want to browse on your SP library to choose the document (URL link) for review? I afraid that there is no way to achieve your needs in PowerApps currently.
As an alternative solution, you could consider manually copy the URL link to document in your SP Library, then paste it within the corresponding field in your Edit form.
For your second question, do you want to keep data fulfilled by User1 on screen 1 when the User2 continues fulfilling another form? I afraid that there is no direct and simple way to achieve your needs in PowerApps currently.
You could consider save the form data fulfilled by User1 on screen 1 into your data source when the User1 finishes filling (via pressing "Submit" button within your Screen1). When User2 open this app, load data from your data source into the Form1 in Screen1 based on some unique column value.
For your third question, do you want to get a notification within your app when you get a approval request from a 3rd User? I afraid that there is no way to achieve your needs in PowerApps currently.
For your forth question, do you want to save your form data into a Word file or PDF file within your SP library? I afraid that there is no simple way to achieve your needs in PowerApps.
Please also check and see if the follownig videos would help in your scenario:
https://www.youtube.com/watch?v=2rfAQe1kK8M
https://www.youtube.com/watch?v=IhE0-HumNNo&feature=youtu.be
Best regards,