OUTLOOK attachments

I understand how frustrating this is, K.R. Luke Campbell

Since the same document could be sent from your iPhone, this points more toward an issue with Outlook on the Mac. There have been quite a few similar reports over attachments in Outlook for Mac, and below are something that have worked for other users that you can try out:

• In Outlook, select Help > Check for Updates > install any available updates.
• Try dragging the file directly from Finder into the email instead of using Attach File.
• Reset Outlook's disk access permission:
  1. Open your MacBook's System Settings > Privacy & Security > Full Disk Access.
  2. If Outlook is not listed, select the + button > add Outlook and enable access.
  3. If it's listed, enable it > If it is already enabled, toggle it off and then back on.
  4. Quit and reopen Outlook after making the change (or restart your MacBook).

If it still fails, as an immediate workaround, you can upload the document to OneDrive and send it as a link instead of a conventional attachment. Other than that, please let us know your Outlook version, macOS version, the details on the files, ... Those would help narrow down the causes of the issue.