Do you mean as user filling the forms or as a developer writing software to fill the forms?
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.
