[SalesForce] Accessing a pdf as content document via URL

I haven't tested it, but how about URL.getFileFieldURL(string, string).

Returns the download URL for a file attachment.

You might need to work back to the entity that has a reference to the attachment in a field.

UPDATE This doesn't appear to work in practice. From the Summer 13 release notes:

We’ve also created a new Apex method, URL.getFileFieldURL that lets you retrieve the download URL for file attachments in custom implementations of Ideas.

So it might be that this method is only applicable to ideas.

Otherwise Dynamic apex/visualforce might help, but you would probably need to change from returning a PageRefernce. See Dynamic References to Action Methods Using $Action

According to this post, the answer to your question about using content packs would be "no".

On the IdeaExchange you'll find a submission titled Allow Content Deliveries to be programmatically generated via Apex which has some related links on it you may also want to look at. Regardless, I believe this "idea" covers the gist of your question.

It seems to me that you might want to go and vote for this idea.

With a bit more effort, I believe I've found something in another post that you might be able to utilize the techniques from for getting a download link in it to accomplish your task. There are references to a blog post titled Salesforce: How to make a Document public? and some other useful discussion.

Based on the above referenced post, I'd say the answer to your final question is "yes". Implementation will be a matter of how you design your trigger and email template(s).

For future reference, you might find FindSF Info to be a very helpful search engine.