DjVu is a file format that usually stores a combination of scanned content, such as images, text or line drawings. This format comes as an alternative to using PDFs for storing scanned text and image content. The difference between the two formats is that DjVu successfully compresses its contents, making it a lot easier to share or transfer scanned data online. Unfortunately, Mac OS X doesn't provide you with a default program for reading this type of files. For this reason, programs like DjVu Reader were created.
This program is more than its name suggests. Besides allowing you to open and read DjVu documents on your Mac, this application also enables you to print the imported files or export them to different formats, like PDF or TXT.
Furthermore, you can choose between multiple reading options or zoom over a page from the imported document.
Sadly, the application comes with several flaws. For starters, you need to make an in-app purchase in order to unlock all of its features (before making the purchase you can read only half of the imported DjVu file). Another disadvantage is that the program occasionally runs slow, especially when importing or printing your DjVu files.
Hence, I'd say you should try a different DjVu reader. You can easily find cheaper or even free solutions for reading DjVu files on your Mac.