I have some PDF and want to have light gray background when viewing them, but i don't need to permanently change its background. What's the easiest way to do that? I normally view it using Preview on Mac OS X, but i don't have to use it.
Otherwise, you can convert the white background of your PDF to whatever color you want, following the procedure I described here. Work on a copy of PDF and use this resulting PDF when you want read it with a gray background. This only works for PDFs with a vector background.
In your case, you change to gray, but the logic is the same for any other color you want to set.
In Adobe Acrobat Reader, you can set a custom background color as part of the accessibility options, provided that there is no background image.
The instructions are for Acrobat Reader 9. Other versions should be similar.