Registration on the forum takes only a couple of seconds (and it can be done here - Register) but if you don't want to register you can login as anonymous by using the username/password anon.
This will let you log in and create a new post/reply without registering. However, if this will be abused (posting spam or inappropriate content), the anon user will be deleted.
Topics shouldn't be started in the Announcements forum (that's for official announcements) and if you have a question about doPDF you should start a new topic instead of replying to a tutorial/announcement/post which has a different subject than your question.