Check the followings to improve the scanned image.

• The Paper Guides are not adjusted to fit the width of the document.
  Adjust the Paper Guides to fit the width of the document. When Scan Size is set to Auto and you are scanning a document that contains multiple page sizes, insert narrow pages straight and as close to the center of the machine as possible. If the document does not stay straight while feeding, use Continuous Scanning.
   
• The Auto Deskew feature was set to On when the document was scanned.
  Set Auto Deskew to Off.
   
• The settings are not suitable for your document. For example, a Legal^*1 Letter/A4 size document was fed when Executive/A5 Letter was chosen as document size.
  Make sure the document size settings are suitable for your document.
  *1: Only available some countries.
   
• A thick document may be scanned from the Auto Document Feeder (ADF) when the Output Tray is pulled out.
  Close the Output Tray and scan your document.