Posting a file to a public web server is an implicit invitation for clients (human or automated) to download that file. That's why "secret urls" are universally considered to provide very little security.
There are common conventions (not always followed) around robots.txt and what files to crawl, but I'm not aware of any rules or conventions or standards around URL discovery. Plenty of crawlers attempt to crawl every registered domain name, for example.
"DOS Attack" is sort of a loaded term since it implies malice. Clearly running a web server doesn't mean you invite malicious attacks (though perhaps you should expect them). Some people consider Googlebot to be a DOS attack since it can easily bring poorly designed sites to their knees.
