Generally, if the item you are scanning is not glossy, then we recommend to use the smartphone flash and the LED lighting (if available).
In most apps, you can tap the screen to focus on a different area of the image. This also corrects for brightness. Therefore, it can help to tap a section that appears to be darker.
To improve uniformity, we recommend to select “auto color” in Adobe Scan (under “Color” which is available after you take an image) and selecting either “Lighten” or “Magic Color” in Camscanner.