My .forward file is |/usr/bin/procmail -f- and mail gets delivered, but there was a bounce to the sender, sending him the output of procmail. How can I prevent this?

Exim's default configuration is set up like this:

address_pipe:
  driver = pipe
  return_output

The return_output option requests that any output that the pipe produces be returned to the sender. That is the safest default. If you don't want this, you can either remove the option altogether, or change it to return_fail_output, to return output only if the command fails. Note that this will affect all pipes that users run, not just your procmail one. It might be better to arrange for procmail not to produce any output when it succeeds.