DLL files are compiled libraries or drivers that store executable data or program data for main programs to load, but users can see what binary data they contain using a hex editor. However, the structure of DLL files means that users typically don't open them like other types of files.
DLL files let multiple programs rely on the same code, and some common Windows programming interfaces rely on them. Many device drivers also use DLL files to store executable code. Because they typically contain machine code, DLL files are difficult to read, but users with a hex editor might be able to find readable text that contains copyright information, messages for error codes or other interesting information.