So, which pdf form filler should we use if we do not want our filled data to be leaked or sold.
As a user, just use Acrobat or Preview.
As a developer, there are lots of options. Check out python pdf libraries. There is also a lot of work going into JS PDF libraries if you want to keep everything on the client.
Actually, this is one area where I’m excited about web assembly. There are so many native tools for working with pdf. Compiling those to wasm and running them on the client opens some really interesting posssibilities.
acrobat is nice occasionally for more complex stuff, but if I can do it in preview I use that.