Are there limitations on which fields can be retrieved for the PDF print layout?

If it's on the record or on a jointed table, you should be able to retrieve it.

There are some limitations. You’ll have to create a custom field that will source the data from the fields that won’t appear in the PDF layouts. For example, if you like to visibly see that an invoice says “PAID”, in our case, we weren’t able to find transaction status for PDF layouts. I just created a customer field that will source from the actual “Status” field. This new customer field popped up in the PDF layouts list.