Use even lighting and keep the camera square to the page. The bright patch is likely glare from a lamp reflecting off paper that isn’t perfectly flat.

What helps:

• Flatten the page as much as possible so it doesn’t catch reflections.
• Use two lamps placed on opposite sides of the paper instead of one lamp from one side. This gives more even illumination and reduces hotspots.
• Put the paper flat on a table and hold the phone directly above the center of the page, with the lens pointing straight down. That keeps all four corners at a similar distance and reduces perspective distortion.
• Be aware the phone lens is near a corner of the device, so center the lens over the paper, not the middle of the phone.

If you want a cleaner result, a document-scanning app can help. These apps use the camera, auto-crop the page, and often boost contrast so white paper and dark writing look more like a scan.