Anyone with experience uploading pdfs to NetSuite via REST API and attaching it directly to a transaction e.g. fulfillment record

not a supported operation in rest api. That said, 2026.1 recently introduced the ability to attach a file already in netsuite to a record, so potentially half of what you want will be available soon

As I recall you need to separate this for uploading the file and attaching it to a record using the doc id. I didn’t find a way to do that but you might use the email capture features. Send the file and the record to NetSuite email, in the email capture script download the attachment and save it and later attache it.