Hi Kieran,
Adding additional fields or making other modifications to the PDF finding will require development. You will have to take a look at multiple places. I am not a developer and can't tell you exactly what changes to make, but I can at least round up the various places you would want to investigate to make changes.
First, it's worth noting that the PDF generation was sponsored by a Canadian institution, and is based on the Canadian RAD archival standard and template / export in AtoM. We had always intended to add additional finding aid profiles for ISAD and other standards, but unfortunately, it never happened.
With that in mind: Finding aids are generated asynchronously from the EAD 2002 exports - essentially, we have XSLT files that take the output EAD XML and transform it into a PDF or RTF file, following the rules of the stylesheet.
So, first you will need to do some testing and confirm that the fields you want are present in the EAD XML. I don't actually know for sure if the RAD-based XSLT that turns your EAD XML into a PDF actually uses the RAD EAD (we do have 2 different EAD profiles - a RAD one, and an ISAD(G) one), so you might want to check both by setting the view page template for your description to the correct standard and then exporting the EAD from the right-hand sidebar/context menu.
If you need to make code changes to the EAD, we recently shared some links pointing out where to find the relevant code in this thread:
The Gearman-based Job Scheduler actually runs the task asynchronously, so it doesn't time out in the browser. I doubt this will require any changes, but the main class for it can be found here:
Now, the XSLTs used to transform the EAD XML can be found here: there are 2 versions (Full details, and Inventory summary), whose differences are described in the docs here:
You can find both of these stylesheets here in the code: