Hi londonbookkeeper,
Thanks for reaching out here. Welcome! It's important that your customers are able to view the statements, sent from your QuickBooks Desktop company. I'd be happy to provide some insight here!
When you send files from QuickBooks, the customers receive an attachment labelled Winmail.dat, rather than a .pdf file. This is due to client's not able to receive .rtf (rich text format) files, which is a known issue for Microsoft Outlook. When only some of your customers experience this, the issue can be related to a problem on the receiving end, with mail and security features. There are a couple of ways this can be resolved as noted here. If the issue persists, please don't hesitate to contact us.
If you have any other questions, feel free to reach back out. We'd be glad to help!