First of all you will need to create a document template described in this link: https://world.mendix.com/display/howto40/Create+and+build+a+document+template

You need to create a microflow which will create an object using an entity that inherits system file document. Once you have done this you can then use the document generator explained in this link: https://world.mendix.com/display/refguide4/Generate+Document

Once done you can use the download document action if you want to view the document.