I do not think that attaching the PDFas an attachment is a good idea. Typically PDF documents are rather large in size, and nothing pisses off a customer more than having to download an email with a large attachment. Also, many free email services such as gmail and yahoo have restrictions on attachment file size and number of attachments.
What I would do is create a page that serves protected files to a user that has already authenticated(logged in) with your website. There would be 2 parts to this, and the first one I have already explained but I will give a simple coding examples.
Login.asp - Your file that handles login a user into your website. This should already be built.
if Request("submit") <> "" then
'validation of incoming data goes here.
'Hit your datasource(SQL) to make sure username and password are good.
'Woohoo user successfully authenticated
if success then
Select Case Request("target")
Response.Redirect("serveProtectedFile.asp") 'This is the file in your email. I will give an example of what this file needs to do later.
Response.Redirect("account.asp") 'This is where you would typically redirect a user. Your login page should already be doing this.
'Doh user didn't authenticate throw error message
errorMessage = "login failed"
<form action="login.asp" method="post">
<input type="hidden" name="target" value="<%=Server.HTMLEncode(Request("target")) %>" />
username: <input type="text" name="userid" /><br />
password: <input type="text" name="pass" /><br />
<input type="submit" name="submit" value="Submit" />