The Digital Library Department of the Bavarian State Library has an incredibly generous online offering for the classical music community:  the complete works of Felix Mendelssohn as digital scores, scanned in high quality for clear viewing.  Scores are available as PDF downloads, but you need to navigate a bit of Bavarian German to assent to their download policies (i presume - my german-reading skills are pathetic).  This is an invaluable resource for serious musicians who can use these scores, readily available online, for the study and research of one of classical music's greatest musical masters.
Check out the image below to see where to click to get the PDF downloads in the score detail page view on the site.