Whether a user is asked to signup/login before downloading a file depends on a number of parameters, including the number of files being downloaded from your account at that time and the type and size of file.

This behaviour is designed to make us unattractive to the people that would want to use our online storage service for illegal file sharing on a mass scale whilst causing minimal impact to our normal users.

Your visitors will only encounter this behaviour if your account is high traffic or you are using large files. The file can always be downloaded after logging in so your visitor will always be able to get the file you offer.

The restrictions are relaxed for premium accounts.