Hello there, @Julie211.
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.
Here's how:
- 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.
To know more about handling documents, you may check out this article: QuickBooks Document Center: FAQs and common issues.
I've included this reference that you may use for your future tasks: Tracking job costs in QuickBooks Desktop. This guides you on how to compare the expenses to your revenue for a job.
Please don't hesitate to comment below if you have other concerns or questions. I'd be happy to answer them for you. Take care.