Ok, you are not going to be able to do it the way you are describing. There is no way for one app to hyperlink to a document in another app. Each app's documents are held in their own (sand boxed) file directories. There is no central file system for the sharing of files.
That's why DropBox is so popular. It's a central repository where apps (that support it) can share files. But it is online. You can link to the URL, but not to the file on the iPad.
I think what you need is a two pronged attack. Store your brochures in the shared folder of DropBox. From here you can get a URL for each files online location. Go ahead and place that URL in the database. Even if there is no direct way of downloading the file using a link in the database (and there may be, I'm too lazy to read that entire manual), you should still be able to copy and paste the URL to one of several programs that can. GoodReader would be an excellent choice.
GoodReader can also go directly to the DropBox account and upload/download files. That way your field people would be able to grab any brochures they need, even if something gets mixed up in the database or they have sudden changes in itinerary.
If you want to force a brochure to the field, you can use the same URL to email the pdf to the individual's iPad. They can then tap the URL to open the pdf in Safari, and then use the Open In feature to view it in one of a variety of apps (if local storage and more features are needed). Again, I recommend GoodReader.
You were talking about scripts. Without knowing anything more about what the database is capable of (on the server side), it may be possible to set up scripts that would place the brochure in DropBox on the server, copy the URL to the database field, and mail the URL/attachment to the field with a single command, or whatever.
What you are not going to be able to do is force a file to automatically download on the iPad and then be linked in another app. Well, you could probably do this if your wrote your own app, but I don't' know of any other way to do it.
It is possible that your database app on the iPad will recognize URL links and let you open them in Safari. I don't know, but it's worth exploring. That would save a step or two.
Thanks alot for the detailed answer.
As I suspected, Dropbox is "sandboxed" as you say so that's a non-starter.
I'm essentially looking at building a sales tool for our reps out on the road, we could have 30-40 new products every day with nice, hi-res images and, as they may not have a wifi/3G connection I'd need them to store the images before they head out on the road. A quick Dropbox sync each morning was the plan but hey ho.
Take a look at the "synch" tools offered by 360works and SeedCode. I saw a demo by a SeedCode rep of a solution they set up using their synch tool for inspecting properties under construction that sounds exactly like what you have described here, the users download a subset of the total data in the DB on to their iPad, they visit the site with that data to tell them about the site and the collect new data about the site--including pictures, which they then synched back to the "mother ship" once they reach a location with a good wi fi connection.