I often discourage people from ‘gating’ content (hiding it behind a registration request), but that doesn’t mean there aren’t sometimes compelling reasons to exchange content for an email address. What is very important to ensure in such situations is that the content actually arrives. Unfortunately if you’re emailing it, that can be out of your hands. If someone fills in a form asking for a document, and you then email that document, it could quite easily get caught up in a spam trap. Emails can also occasionally have problems getting through first time, especially from automated mailers. And finally, if your automated system fails, it may be a long time (and many frustrated prospects) before you find out it’s not been working.
Ideally, if someone fills in a form asking for a document, you want to ensure they get it straight away. How do we do this? Give it to them straight away!
Your online form says: “Complete your details and we’ll email you the document”. When they do so, they’re taken to a ‘thank you’ page, and the document is (with any luck) emailed. But why not put a download link to the document on the ‘thank you’ page too? After all, the page will be off-grid (only accessible as a form response). The only explanation you need should be something like: “Thanks for requesting the document. We’ve emailed it to you so you’ve got it for safe keeping, but in the meantime, you can also get a copy by clicking here.”