This is a good start -- http://x-callback-url.com/ is a standard and listing for apps that support it. The biggest problem is that there is a limited amount of data that you can transfer in a URL (about 80KB I believe) so even large PDFs will start to cause problems.
See "communicating with other apps": https://developer.apple.com/library/ios/documentation/iPhone...
In one app framework we built we had a fairly complicated parser and it could handle a pretty wide variety of URL paths and the supplied data. But there in turn lies the problem - each app must have a mechanism to handle the parsing and routing for these URLs.