PDF documents do not let you edit them in any way, though let you add some comments and tips to the document. However, Adobe Reader allows you to customize the document colors and change them o high contrast themes if you want to read them at night. For example if you want to read a document in darkness at night, then setting the background from white to black becomes a bit more soothing. The following tutorial will help you Invert Colors in a PDF File for better night time reading.
There is one drawback for the feature. Once you apply it, it applied for all the documents you open using Adobe Reader. So you need to revert back to the original settings once you are done!
1. Open a PDF file. Now go to Edit and now click on Preferences.
2. Now click on Accessibility located in the left panel.
3. In the right panel, under Documents Color Options, check the option Replace Document Colors.
4. When you check the Replace Document Colors option, now check the option Use High Contrast Colors. Choose the color combination from the drop down box.
5. Click OK to close. The text colors have changed.