When an email attachment is opened from within Outlook for Mac, it's opened Read Only from a temporary location. While you can open, view and print the attachment, you cannot modify the original copy Outlook received from the sender. Even if you don't wish to modify the file, you may wish to save a copy of the attachment somewhere else, such as OneDrive.

For more info, see Microsoft's help article: View attachments in Outlook for Mac.

Save attachments directly from Outlook

1. In Outlook, click the downwards arrow at the right of an attachment, and choose Save As...
   
   
   

   Tip: You can save multiple attachments by holding the Shift key and clicking icons to multi-select. Click the downwards arrow for any one attachment, then Save as... All selected attachments will be saved together into the same location.

   
   
   
   
   
2. Choose a permanent storage location for the attachment(s), such as your OneDrive sync folder, and click Save.
   
   

   Optional: If you've saved the attachments elsewhere, such as a workgroup share, you can use the Remove or Remove All options to remove one or all attachments from the email. The email itself remains. This can help you save space in your mailbox.

   
   
   

Save a copy from within the application

If you've already opened the attachment into its parent application, you can save a writable copy from here.

1. With the document open in its parent application (e.g. TextEdit, Adobe Acrobat, Microsoft Word), click the File menu and choose "Save As..."
   
   

2. Save the new file to a permanent storage location, for example, your OneDrive sync folder.
   
   
3. Open the newly saved writable copy from the location you saved it.

Some applications may have their own features for dealing with read-only files. For example, Microsoft Office apps display an alert bar suggesting you make a modifiable Duplicate. This copies the contents of the file to a new document, which you can then save, along with any changes.