I have an A4 (297mm x 210mm) layout in indesign for print with my images in 300dpi. I would also like to make a pdf for people to read on their devices (eg. android, ipad etc.). Do I need to make a seperate document with different dimensions and dpi of images for use as a pdf on devices? thanks
No. But you may need to export two different PDFs. One for printing and one for on screen viewing.
If your document is difficult to read on a smaller screen you may wish to redesign it for (say) tablets. Also you can use the features of a screen version such as hyperlinks.