QB Issue Resolution:
There is a bug in El Capitan, restore your old OS, print to .pdf and attach from Mail, stand by until Apple/Intuit releases an update.
Resolution for Issue 'Why is Quickbooks Mac not attaching estimates and invoices in emails to customers. This problem began when I download the new OS El Capitan.' available: Yes (Solved).
Source: Intuit Community forum.
Why is Quickbooks Mac not attaching estimates and invoices in emails to customers. This problem began when I download the new OS El Capitan.: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.Support for Appleupdate
Quickbooks Repair Pro will resolve it efficiently and affordably. Guaranteed resolution of your issue, or you pay nothing!
Error Recovery for Appleupdate [Canada]
E-Tech is the #1 Quickbooks data migration and Conversion service provider.
Additional Support Options: