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They may be a need to update your Adobe Reader to the latest version, mignonbonino.
Once your PDF viewer is updated, set it up as a default application to open PDF documents.
You can check this article for more information: How to update, repair, or re-install Adobe Reader/Acrobat.
On the other hand, you can also check the setting in QuickBooks if there were changes made.
Let me know if you have other questions about emailing invoices in QuickBooks Online