Our client is having exactly the same issue as the original poster and the problem still exists 3 years after the original post.

We do not have any issue attaching files to invoices. We have spent hours and hours over the phone with different QB support teams, repeating all the troubleshooting steps in the above posts, and fixed nothing. Because the issue has nothing to do with the Document Centre.

The issue is when we use "Email" --> "Invoice and attached files," it doesn't allow to send attachment over 2MB. We have to remove some of the attached files in order to send. The attachment size limit of 2MB is too small and dated to meet the current IT environment. 

Our client is a contractor in the construction business. The industry requires them to attach mandatory forms, certificates and any material purchase receipt every time they invoice their customers, which often times can be over 2MB per file. The client has to save the invoice to their file server and send the invoice and mandatory files out via their email program, creating unnecessary steps to their workflows.  

Please increase the attachment size limit to meet modern IT needs.