Will the recipients of that Pages document have access to Pages, or is it strictly an MS Word audience?
If the latter, export the Pages document to Word, and save to your Desktop, and then tell Outlook to look on your Desktop for that Word document to attach ... or, drag and drop the Word document onto your One Drive, and attach from there with Outlook.
I suspect that you are using an unexpanded Save/Save As.../Export to dialog that runs home to iCloud because it remembers that was the last save location.
If your default panel from the last pararagraph looks like the following, then click on the arrow in the red circle to expand the dialog, and choose your Desktop, or local Documents folder, instead of iCloud Drive.
