In SuiteCRM88 , when creating PDF templates, related modules linked to Products are not displayed. There is no option to select values from related modules created in the Module Builder. I have already performed a rebuild and updated the relationships.
Generally, fields from related modules do not work in SuiteCRM PDF logic. Only use fields in the current record. If you need a field from a related record, create a custom field in current record and populate it by workflow. This will allow you to use the data in the PDF.
Appearently, there are some improvements coming down the road to allow this kind of thing.