Thanks for your hints! I added a 'Open PDF' button to circumvent popup blockers.

Yes, I used emscripten to port it to Javascript. It was not that hard. Emscripten had three bugs I had to fix (the hardest was to find that the %g format was not supported by emscripten's sscanf).

But it was compiled almost like for x86: first convert the pdftex 'web' souce code to c using web2c, then compile it to LLVM bytecode and the LLVM bytecode to JS.