But I don't know how to change the C/My folder/2020/File.XFDF to The User's Document folder/2020/File.XFDF?
Yes, you can get it using this code, but you have to run it from a privileged context:
Today I worked on it and it and used a variable for the user documents map and filename. Then checked it with the app.alert function. And then put the variable after cPath and it workes fine. (Windows 8.1).