The process is pretty "easy", with a few caveats.
1. XML files can be converted to PDF and/or HTML by using an appropriate XSLT file
. By default, OJS comes with an XSLT file that is compatible with NLM 2.3 XML
, a fairly popular XML journal publishing format. If you want to use another XML type, you will have to either download or create an XSLT file appropriate to that XML type, and then upload that XSLT to OJS from Journal Management -> System Plugins -> Generic Plugins -> XML Galley Plugin, where it says "Custom XSL Stylesheet". Note that you will need a different XSLT for each conversion to HTML and PDF; and that currently, the XML Galley plugin only allows for one custom stylesheet to be uploaded, which means that if you are not using NLM XML, you can only convert to PDF or HTML, not both.
2. XML->PDF conversion also requires an external application, installed on the server, to work: a Formatting Objects (FO) processor. I'm not sure that we've installed any such processor on our test install server, so you may be out of luck with respect to generating PDFs on that particular install.
With those caveats out of the way: to convert XML to HTML or PDF, all you have to do is enable and configure the XML Galley plugin; and then upload XML files as your galleys during submission editing. They will be converted to the appropriate file type the very first time they are viewed online, and the converted file will be stored in OJS for future access.
If you have any further questions, please let me know!