Hi. If this is a document that you produced on a local device that device should have software or hardware controls that affect the brightness of the scan. If youre using a mobile app like CamScanner s there will be controls but you can also adjust the lighting to get a better picture. More daylight (but not bright sunlight) is usually better. If it a third-party PDF then your options are more limited. Depending on your software you may be able to export save the PDF as separate JPG files (one per page) and use an image-handling app like Lightroom to adjust the brightness contrast. Again depending on software you would then be able to reassemble the pages into a document. Worst case you could try printing the document and adjust your printer settings to give a darker output. Then re-scan it into a PDF file. Adobe themselves don seem to have many options on offer.. Low contrast on scanned PDF s