When you create your PDF if you have that many pages it will take quite some time. Where are you saving your file? I usually create the PDF in my Temporary Folder when I just want to sent as attached file in a email. FileMaker does not do the next step until the script before is finished. You can send email through your email client or directly using SMTP settings
Also i use a temp file but & yes I use SMTP but it does look like filemaker isn't waiting on the file to be created to the disk, when i pause for 5 seconds it works fine.
This is working now so its probably not the end of the world as the script will be automated and the delay won't be an issue for the user.
also I can't rule out that its the email server holding up emails, so thanks..