I'm happy to assist you with the attachments in your email, but first, I need you to clarify a few questions for me.
Does the attachment appear on the Invoice preview before you send it?
Can the customer see the Invoice in the email?
Can you provide a screenshot of the email?
By answering these questions and providing a screenshot, I can assess the issue and give you steps on how to fix it. In the meantime, I recommend having your customers make sure their Adobe Reader is updated to the newest version. This will ensure they can open and view any attachments you send in an email.
Since you've seen the attachment in the invoice before it's sent. I'd suggest trying to send it to your own email to check if you can see the attachments. This is to verify if you need to change your company settings.
If you're not able to see the attachment in the sample invoice you've sent in your email, you'll have to change the set up of the Sales Forms Preference to either WebMail or Outlook. This is done to successfully make the attachments show in your customer's email.
Go to the Edit menu at the top menu bar.
Click Preferences on the drop-down list,
Navigate to the Send Forms and click My Preferences.
Select Web Mail or Outlook.
Once done, click OK.
I've added some screenshots for additional reference:
However, if you're able to see the attachments of the sample invoice in your email. You may advise your customer to do a browser troubleshooting if it's still not working after updating the Adobe Reader to its latest release.