Hi Adrian,
If I understand correctly you have 3 tasks:
A for asking the user if he wants to see the PDF or go immediately to the conformation
B view the PDF
C confirm the vendor
If your workflow notification settings is to send a mail for every task, lets say your user received a mail about task A and decided to go to the confirmation screen but hasn't confirmed the vendor yet, he has now task C in his inbox and he will receive an email about it since this is a different task.
You can solve this either by limiting the selection of tasks to be sent by mail only to task A (transaction SWNADMIN or SWNCONFIG - pay attention before making changes - this are usually system wide settings).
Or you can shorten your workflow only to task C and have the PDF display as a link in the task (attachment or an attached object with the default method displaying the PDF). This will also give your users the option to view the PDF whenever they what as long as they haven't confirmed the vendor.
Ronen