If you only need to encrypt a few files and don't want to install an app, Whisply can help.
Whisply comes from the same company that makes BoxCryptor. It lets you upload several files, select how secure you want them to be and then choose a cloud storage service to upload it to. Whisply claims it doesn't have a file size limit, but it recommends avoiding more than 10MB uploads.
Of course, if you need to be extra careful with your encrypted documents, you might not want to upload them to a third-party service. However, this fills a handy niche for those who need to keep their files safe but aren't familiar with how encryption tools work, or if you need to encrypt a file while you're away from your desktop. It can also serve as a helpful introduction to encryption in general.