Here's the thing. I have a 300 DPI image ready for printing. I need a small version of it to use in a digital book. When I say small I mean two things: I will reduce its display size and I want it to become a small file. In Photoshop I have some options.
Change DPI without resample the image: It doesn't help because the file size remains the same.
Change DPI resampling the image: It will result in a small file size but the pixel dimensions will reduce more than I want it to. For example, an image with 2000 x 2000 will became something less than 800 x 800.
Don't change DPI and reduce the pixel dimension directly: Ok, now I can get it the size I want BUT I don't know if this is right thing to do. The image will keep its 300 DPI but the pixels will be reduced. Is it right?
Change the document size: I think the effect is the same as the third one above. Size and pixels will be reduced but the resolution won't change.
We always read that we don't need 300dpi images for e-books and web and we should use 72 instead. My ONLY concern here is to produce a small file that will cause my e-book to have an acceptable size to be downloaded. So, I don't care if the resolution remains 300 DPI, as long as the final size is reduced. So, the third and fourth option seems to to that, but I need a more advanced opinion.