I wrote some C# code a couple of years ago to generate PDF invoices. Has been running fine since then generating tens of thousands of PDFs. It was pretty straight forward back then, only annoying thing were page breaks when there were a lot of line items, but apart from that I would not consider it a "pain".
What issue have you had? What do you mean by "custom"?
The pain in there is that every time you want to have a PDF generation system, you have to do the same stuff, it's not "complex" at all, but it definitely takes time, whatever the technology you use. On my experience, having ten thousand PDF generating at the same time was a very high load for our infrastructure and the cost was enormous, that is one of the main thing I'll focus on today : the scalability of it. It also depending on the way you create your template, hard coded with pixels or HTML!
You're right, per se generating a small number of PDF is indeed easy.
What's complex is:
- Dynamic PDFs: you're trying to digitize a government process for instance, by launching online forms with a variety of user paths for the answers and generating a PDF with the answers that still fits specific government requirements
- Doing it at scale
What issue have you had? What do you mean by "custom"?