The Rochester Public Library recently added the 1872 Atlas of Livingston Co., NY to their collection of digitized books. This atlas is in PDF format. RPL has previously used a software package called Zoomify to present maps and atlases. (See this web page for those available maps.) I like the PDF format better mostly because Zoomify uses a small window to view a large map. The atlas as a PDF can be as large as your monitor can display.
My great-grandparents lived in the Town of Portage, which is the southern part of Livingston County. Unfortunately they moved there about 1880. This atlas is too early to show where they lived. I did find a distant relative, Nelson Saxton, in the Town of Portage. He is my second cousin, three times removed.