If you have a webserver or cloud storage you could store your pdf files there, then in your SQL Server database you could store only the URL's to the pdf files.
Don't try to do this with Dropbox, it is no longer possible to share URL's in a simple way.
If your files are not too large you could retrieve them like this:
using(WebClient client = new WebClient()) {
string s = client.DownloadString(url);
}