download PDF files from website

I am trying to automate the download of several PDF documents from a secure website that requires a login.  The website uses https and requires a login (which of course I have).

When I try to make WebRequest GetResponse calls it fails with a 401 error - Requires Authentication - even though I can navigate to the PDF form using the WebBrowser1.Navigate("[URLOfPdfFileHere]") method.  "Stuff" must be happening at the server side, which means there is no direct URL address for each of the PDF documents - so, typically, when clicking manually on a document link the URL would be shown in the status bar of the browser as:
https://www.theirsite.com/subscription/cheatsheets/NXT/gateway.dll?f=id$id=cheatsheets_form_1_pdf$t=document-frame.htm$3.0$p=

I have tried the following to get this working:
Dim oDownload As New System.Net.WebClient
URLReq.Credentials = New NetworkCredential("MYUSERNAME", "MYPASSWORD", wb1.Document.Domain.ToString)
oDownload.DownloadFile(https://www.theirsite.com/subscription/cheatsheets/NXT/gateway.dll?f=id$id=cheatsheets_form_1_pdf$t=document-frame.htm$3.0$p=", "C:\test\cheatsheet1.pdf")  ' ------FAILS HERE WITH A "401 ERROR - REQUIRES AUTHENTICATION"

When setting the Credentials, I used the "wb1.Document.Domain.ToString" value because I figured that whatever network credential WebBrowser1 was using must be OK - since I was able to navigate to the forms with WebBrowser1.  I have also used:
oDownload.Credentials = System.Net.CredentialCache.DefaultNetworkCredentials
...but this too did not work.  Same "401" error.

One idea I tried was to actually one by one navigate to the link for each document and then when the browser loads the form (within an Adobe Reader frame) programmatically do a "Save As" to save the form to disk.  EXCEPT - I then cannot get a handle on the Adobe Reader frame within that webbrowser control to do the Save As.  If I execute the following:
WebBrowser1.Navigate("https://www.theirsite.com/subscription/cheatsheets/NXT/gateway.dll?f=id$id=cheatsheets_form_1_pdf$t=document-frame.htm$3.0$p=")
...then the PDF file opens up within WebBrowser1 - but then I cannot figure out how to get the file out of the WebBrowser (manually, I would simply hit the save button in the WebBrowser's Adobe Reader toolbar, but I am trying to automate this to not require user input).