To edit the content or properties, such as the contrast, of a PDF, you need a PDF editing application. To lighten or darken a PDF, use the PDF editor to adjust the contrast on the document. The premiere PDF editor is the Adobe Acrobat program, and though it tends to carry a hefty price tag, Adobe offers free trial versions of Adobe Acrobat Professional. Download and install this trial version if you do not have access to Adobe Acrobat or another PDF editing program.
Right-click on the PDF document you want to darken, and click "Open With."
Select "Adobe Acrobat" from the context menu. The document opens in Adobe Acrobat.
Click the "Document" option on the top navigation bar in Adobe Acrobat.
Click the "Background" option. The Form Background dialog box opens.
Click the "Add//Replace" button.
Click the "Form Color" option, then click the color swatch to launch the color picker. Select a darker color for the background of the PDF, if desired. If you already have selected the color you like for the background, adjust the Opacity to show 100 percent saturation of the color.
Click the "Opacity" option to change the opacity of the PDF background. Move the slider to "No Opacity" to show 100 percent saturation of the color for the background. The PDF is darkened.
Click the "File" option, then click "Save" to save the changes to the document.