All readers, including those for PCs, download a copy of the file to be read, only saved in the temporary browser directory (cache). There is no such thing as reading how the videos are streming.
You can intercept the WebView_OverrideUrl (Url As String) event and if the url ends with ".pdf" display the file as you prefer, with a reader, with a library (ce There are many for b4a) or how do you think
Sub WebView_OverrideUrl (Url As String) As Boolean
If Url.EndsWith(".pdf") Then
'Download file
'Display file
Return True
End If
End Sub