Attachments in email
How do I make an attachment larger than one third of the screen? (Column on right side of screen)
iPad, iPadOS 13
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You perhaps have a Slide-Over view enabled, this being a feature of iPadOS.
The easiest way to switch from Slide Over to a full-screen window is to touch-and-hold the small drag-bar at the top of the Slide Over Window, drag to the top-centre of the screen - then release; this window will become full screen.
Here is an Apple video that demonstrates use of Slide Over:
... and the corresponding page from the online version of the iPad User Guide for iPadOS:
https://support.apple.com/en-gb/guide/ipad/ipadfe7c65e9/ipados
... and a further help page that describes many associated features of iPad multi-tasking:
https://support.apple.com/en-gb/HT207582
I hope this information os helpful is resolving your immediate question - and in providing useful references to significant functionality.