Howdy Ya’ll! I am back!! Haha! Missed you!!
An app (if it were my app) populating an existing PDF, of someone else’s, using Zapier or another set of additional connections sounds complicated with way too many steps and too much margin for error for me.
I have basically done this same exact thing, only for all of the (massive) data onboarding capture needed to set up China/USA/International full container shipping including Customs and Brokerage documents, IFS Agent, Power of Attorney agreements, etc, etc… I found that the PDF documents and amature-hour spreadsheet forms provided (by one of the worlds leading international shippers, Yikes!) for me to fill out the same contact data & info over and over (that I had already given them to set up my account with them in the first place!) was a borderline insanity process, rife with opportunity for very costly errors! Very costly, indeed! Haha! Anyhoo, it was easy for me to recreate every document and form in Google Docs as a Template for my app to automatically fill out with a simple Workflow. @Fabian is correct, you will want to use Tables in your google Doc Template.
My App/this process a huge time-saver and error eliminator, Client data is entered once and app automatically generates all documents & agreements, including signatures (with Timestamp & GPS stamp at point of Signature) for each document & agreement.
In your case, the W-2, for example, looks like a super easy-as-pie thing to perfectly recreate using lots of Tables in a Google Doc. I’d think you should even be able to get it just the correct size.
I do like the FDF/PDF ideas that @MultiTech_Visions and others mentioned if it could make this simpler, but I still see lots of room for error if your are trying to automatically fill out someone else’s PDF. This whole onboarding Forms/Documents & Agreements requiring signatures should be huge for AS. It’s literally crazy to -have to deal with- see how much time, effort & money is wasted in the World by using a Haphazardly cobbled together minefield of data gathering mechanisms.