Having gone through visa processes about 8 times in 3 countries who are Schengen border members, I can tell you that changing the purpose would likely send up red flags. Based on my experience, I think you would have to start the process over again. If you think it will be rejected, then canceling the current application, then starting again from scratch, is probably best. However, it sounds like you’re close—why not just get the conference to send you and the consular services a printed copy of the invitation?

Do you have email confirmation of registration? If so, ask if you can forward that to them. It could be enough—you need to phone or email and ask if that will suffice. Explain that conference invite came as .pdf and you are concerned about the time it will take to get a paper copy to them from the conference organizers. Forwarded email shows a trail of accountability (yes, I know email siggys can be forged, but it’s easily investigated).

Thank you Don for your well explained reply.
As you said, I will ask them to submit email confirmations and related documents like letter of acceptance for my paper. I know my application is true and the purpose is clear. I fulfilled all other requirements and hope they understand my situation. But if they don’t, well heads up…life continues.
best regards,

If there were other problems, they probably would have rejected it already on other grounds. The fact that they only need this paper is a good sign—based on my experiences. Bureaucracy is bureaucracy everywhere, but at least, generally, I feel like I was always treated fairly and equally in the visa process each time.