Hi Kevin Drew (kdrew),
As per the description, it seems like the user has already setup Outlook as the default email program but still it shows the error saying needs to set a default email program or General mail failure error. If my understanding is correct, could you please confirm if the former error you mentioned is same as the error in this MS article: No default mail client error - Microsoft 365 Apps | Microsoft Learn saying "Either there is no default mail client or the current mail client cannot fulfill the messaging request. Please run Microsoft Outlook and set it as the default mail client."
If so, please try the official suggestions from the article and make sure to back up the registry first. And also, I found some user experienced the same scenario while they did manage to fix this by editing the registry key. I have attached a screenshot below for your reference.
However, I appreciate your understanding that sometimes initial suggestions may not solve the problem very soon. But we can work together to reduce and resolve the situation. Therefore, please be patience and provide more information back with me so that we can continue working. For example, please create a brand-new local user profile and see if in a new environment the File>Share>PDF function will connect to Outlook successfully. Reference: Manage User Accounts in Windows - Microsoft Support
Please correct me if I made any misunderstanding and I will be here waiting for your updates to offer any further support.
Appreciate your time and patience. Hope you a nice day and stay safe!
Best Regards,
Mia