You are missing a quote and an ampersand (&) in your calculation.
This puts the / character outside the quotes and FileMaker then interprets this character as the division operator and you get null divided by null which produces the ? result.
Here's a corrected expression that won't produce the ? result but it is still up to you to verify that the resulting path is valid:
"filewin:/C:/Users/TEMP.FLUID.000/Documents" & "/" & Invoice " & Invoices::INVOICE_ID & ".pdf"
That worked great, Thanks.
Also, I'd like to "automatically" send the pdf via email, is SMTP the best/ or only way to do that?
By automatically, I mean without user interaction.
Send mail can use the user's client email application to send out such an email with your PDF as an attachment. This need not involve any user interaction, but you do have to check Filemaker Help for what apps are compatible and see what client application is installed on the user's machine if you choose to use that method to send out email.