sometimes when I’m reading a long pdf file or a document online, it is usually white background with black text, which is very hard on the eyes, is there a way to temporarily change the background color?
Online: in Firefox, go to preferences. Content/Fonts & Colors/Colors. There both can be changed ( and will apply until changed back ). Other browsers must have the same.
In Fonts & Colors, size of print can be increased or decreased.
I don’t think pdfs are meant to be altered. I’m using Oklular, and in settings there is an Presentation tab, but changing the colours has no effect on existing documents.
I don’t know of a way to do this for PDFs, but I think it would be an excellent feature to have in a PDF reader (and a word processor). So far as I am aware, none of them have it, but I do not see why it would not be possible for someone to program the feature in.
I always keep my word processor to display black text on a desaturated light blue background, which I find much easier on the eyes than white. However, even this takes a bit of effort and ingenuity to achieve. I do not seem to be able to set the blue background just in the program, but must make it the default window background color in Windows. Even this workaround does not help with PDFs, however.
At least browsers do allow you to set a default background (and text) color, and (in some circumstances, at least) you can override the defaults set by the web designer, but even there, in Firefox at any rate, the choice of colors is rather limited and none of the choices provided really match my preference. There are Firefox addons such as Tranquility (which I use) and Readability (which I used to use before it turned into spyware) that will enable you to change background colors and fonts too, but even with these there are lots of limitations, many pages do not display acceptably, and the range of options provided is not very large.
Barring any better methods, the pdf viewer I’m using, PDF-XChange, lets you draw boxes over the document. If you set the blending mode to the correct one (it uses multiply for highlights), it’ll be the same as changing the background colour. This change can be saved into the file, but is still removable. PDF-XChange is free.
If you are using Windows, you can also use the magnifier program included with Windows to invert the colours of the whole screen.
Start Magnifier, in the “Views”-dropdown, select “Full screen”. If you then use the key combination CTRL+ALT+i, the screen will be inverted (so it will be white text on black background, and the Straight Dope banner will be a blue on a rather unpleasant yellow/green background)
This page appears to show how to change background color in a pdf: Techno Dys: 4. Background colour - Acrobat Reader
On some PDFs you can highlight the text (as if you were going to copy and paste it). That’ll change it from black on white to white on black. Of course, for most people, that’s harder to read.
Typically, they only time I do that is when I’m reading a webpage that someone typed with really bizarre colors. Like a blue background with yellow writing, so I’ll highlight it all and hope for something better (and with my luck I’ll get red on green).
Well what do you know? That works, and in PDF-XChange viewer too. 
AaronX, I am a PDF-XChange user too, and I can see how your method would work, but applebetty’s way is a lot more convenient to use. I have explored those preferences menus before looking for something like this, but they are rather cryptic and I did not recognize this facility for what it was.
Hmmm. Further experimentation reveals that applebetty’s method works on some PDFs (maybe most), but not all. I have got one that stays stubbornly white, even though others now load up as blue. I am not sure what the difference is. AaronX’s method still works, but is even more awkward to use in practice than I expected.