Rebrickable does not store copies of the LEGO instructions, but there are several other sites where you can find them. On a Set's page, there is a tab where you find direct links to the instructions, provided by LEGO.com
Brickset - provides links to the PDF files stored on the LEGO servers. Unfortunately, this only includes sets that are fairly recent as older ones are removed from the web site once they are discontinued.
Brick Instructions - have page by page individual images of all instructions, including older ones.