I Replaced a $40/mo PDF API with 200 Lines of Web Worker Code — Here's the Offline Invoice Tool I Built

The bill that started this I was paying $40/month for a PDF generation API to power a tiny internal invoicing tool for a client project. Forty bucks a month to convert some JSON into a PDF. That's it. That's the whole service. I finally sat down on a Saturday to see if I could kill that subscription. Three weekends later, not only did I kill it — the replacement is faster than the API ever was, because there's no network round-trip at all. This post is the log of how it went, in the order I actually hit the problems, not the order that makes me look competent. Attempt #1: jsPDF on the main thread (it worked, until it didn't) First pass was the obvious one — jsPDF running directly in the click handler: function generateInvoice ( data ) { const doc = new jsPDF (); doc . text ( data . clientName , 20 , 20 ); data . lineItems . forEach (( item , i ) => { doc . text ( ` ${ item . description } — $ ${ item . amount } ` , 20 , 40 + i * 10 ); }); doc . save ( ' invoice.pdf ' ); } Fine for a 3-line invoice. Once I tested with a 40-line-item invoice (a real client sent me one to test against), the tab froze for almost two full seconds. Not crashed — frozen. Scroll didn't work, buttons didn't respond, and on a mid-range Android phone it was closer to five seconds. The main thread doing synchronous PDF math while also being responsible for painting the UI is exactly the kind of thing that looks fine in a demo and falls apart the moment a real user pastes in real data. Attempt #2: move it to a Web Worker Web Workers get talked about like they're this exotic tool for WASM and video processing. They're also just... a really good fit for "expensive synchronous work that a user is waiting on." I'd never reached for one before this project, mostly out of habit. The tricky part isn't the worker itself, it's that jsPDF assumes it has access to document and window in a couple of code paths (font metrics, mostly), which don't exist inside a worker. I ended up switching to pdfkit compiled