The best idea IMO is to store the path, image/file type maybe even the mime-type in separate fields in your DB.
1: Easier to manage. You can see at a glance in your DB what's "in there" - something that's kinda hard to do with a load of binary data in a BLOB field
2: Less memory intensive - loading a load of binary data into a large field like a BLOB type takes up a load of memory. Compare that with a few strings of text in a VARCHAR field...
Regardless - storing it either way - you should be able to download the document in the appropriate format. Make sure the right mime type is beng sent with the link-click.
PDFs are: application/pdf - so in PHP you would use header("Content-Type: application/pdf"); You may have to futz around with it to get it to work though... you always do...
See this page - I've found it very useful in the past.