Scanned documents appear to be saved as PDF files. Atleast that is the format used when sharing to Mail or saving to the Files app.
Here are a few options that may work for you.
Firstly you can download a free app such as ‘PDF to image’. Then when you long press on the scanned document and press share > copy to PDF to image it will instantly save the file as an image in the Photos app. Each page of a PDF will be a saved as a seperate image.
Otherwise you can always use the “take photo” option in notes app instead or take a screenshot of the scanned docoument when in fullscreen. Alternatively you could use a third party document scanner app which allows you to save directly to photos app.