I apologize for any inconvenience this has caused you. I hope that one of the 2 options that I will outline can help you accomplish your goal more effectively.
The first option is to upload your document to your library, click on the name of your document within the library screen. Then you will need to copy the event URL to place in your email to registrants. This will effectively allow the recipients to download the document. Here is a link to show you how to get this URL for your document. Click Here For Instructions
As an alternative you can export your registrants to a file and import them to their own list. Once this is done you can create an email campaign separate from your event and send to this list. This method would allow you to include a document link in your email without having to manually copy and paste the URL. I have included a few links below to help with this process.
I hope that this helps! I will be sure to submit your feedback to the appropriate department for further review.
We will be here to help if you have any further questions or concerns!