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, you'll see a link in the sidebar on the right which includes links to external resources including Instructions.
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.