Thanks for reaching out to the Apple Support Communities, KhaledAlbably!
It sounds like you’re wanting to know how to scan your documents and maintain the A4 size when you email them and they are opened with Adobe Reader. Check out this thread from tom9699 with a similar question about Exporting scanned documents - Apple Community. This thread has a reply from Apple Community Specialist Allen A you’ll want to take a look at:
"It sounds like you're using the steps from the resource Use Notes on your iPhone, iPad, and iPod touch to scan a document into Notes, and you're then using Share > Create PDF; is that correct? I've tested this by scanning a few different documents into the Notes app and exporting with the same result, so this looks like normal behavior when scanning a document into Notes and then exporting the note. You can always send feedback to Apple here if you'd like: Product Feedback
You may also consider checking out third-party scan apps: Download apps and games using the App Store"
Cheers!