Are you looking to darken the PDF for reading in dimly lighted areas? If yes, you can find PDF reader that have a dark reading mode or a accessibility feature that will allow you to change the view to make it darker. Foxit PDF Mobile has the dark reading mode feature and Foxit PhatomPDF for Windows has the feature located in File->Preference->Accessibility -> Replace document colors. If you want to make it darker in general, that might be a harder task. PDF are made of many objects and you cannot easily darken all of the object at once. Typically you would need to darken each one by one.
Darkening the PDF could not happen automatically by software or in hardware like Adobe Acrobat Reader. It will be your responsibility to find a software that will be able to convert PDF to black and white. However, if you will look after the quality of the text, it will be easy and just your job. But if you want to create a dark PDF, you have to start with the process, and make an important study on how to darken this document. How to Darken PDF? As this topic does not have any easy solution, I will guide you for the right way in the following guide. Step 1 : To get started, you have to download the software that will help you to darken PDF for reading properly. First, download Fox it PDF Mobile or Fox it PhatomPDF for Windows. If you do not want to use Adobe Reader.