To compress a scanned document, see Reducing Document File Size.
If you want to alter, enhance, or otherwise adjust a scanned image or document, look under the Edit menu to find these options:
Rotate & Deskew Page Automatically detect and adjust rotated or skewed pages before or after OCR. To rotate or deskew multiple pages in a document, hold down the Shift key and click to select pages in the Sidebar, or click the first page of the document and choose Edit > Select All to select all pages. With multiple pages selected, Rotate & Deskew Page will switch to Rotate & Deskew Pages. For whole documents, hold down the Option key to switch from Rotate & Deskew Page to Rotate & Deskew Document.
Adjust Image… For images, not for pages or documents. It opens the Image Edit panel which enables you to adjust the color (exposure, contrast, saturation, etc.) of the image as well as apply effects. Go to Edit > Adjust Image…
Resample Image… Reduce a file's size by changing the resolution and reducing the depth of color. Select a specific dpi or switch the color to grayscale or simpler. Go to Edit > Resample…
Deskew & Adjust Image… Make fine adjustments to the “skew,” or tilt, of the image, page, or document selected, as well as adjust contrast and exposure by choosing Edit > Deskew and Adjust Image…