no clean solution for you. you have some workarounds, but the results will not be exactly what you're looking for.
it is possible to make the webview load from cache, but you will have to download the document at least the first time.
the only way to open a .pdf file is with an intent, which means the document will be seen in some sort of .pdf viewer, not a webview. you could download the document in your browser and include it in your app's assets folder. you could then run your intent to view it. in other words, the app never has to download the document.