I think I might be underthinking this being way too simple.I'm in odoo 14 and I've came across a reference online which said there's a default "pdf_viewer" widget in odoo.
So, I added a button with the following code for Delivery Slip, which obviously, only downloads to a pdf when actioned but doesn't show a preview.
Is there anyway to get a print preview easily? Otherwise I'll leave this to our programmer.
Changing the report type to HTML will helps ?
Thanks everyone, I don't have enough Karma to select answers unfortunately, but thank you all