The Web Shield is a localhost proxy that scans http port 80 traffic, when your browser wants to view a web page it is retrieved and scanned by web shield and if of then displayed in your browser, so there should be no double scanning, web shield intercepts locally before arriving in your temporary internet files, browser cache.
I don't know how your download manager works, if it is using the http protocol on port 80 then that traffic will also be scanned in the same way, so again it shouldn't cause double traffic. Are you viewing the pdf files on-line as well as downloading them.
The provider if it has a bandwidth limit it will be logging every item downloaded, e.g. pages viewed, images, etc. they all have to be downloaded to your browser cache before they can be viewed. So you have to consider this as well as any files you download as it all contributes to the bandwidth total.