PDF (Portable Document Format) is a versatile electronic document format. It was initiated by Adobe and was originally intended for electronic presentation of printed materials. Hence, the tasks of this format follow - storing information about the color scheme and the arrangement of elements, as well as ensuring identical output on different monitors and printers. PDF documents can be opened using the free version of Adobe Reader or a web plug-in that is available for most Internet browsers. In some browsers, in particular, in Google Chrome, this format is supported even without the plugin installed. You can edit PDF files using the paid version of Adobe Reader. Also, there are a huge number of programs for both opening and working with this format, and for converting to other formats.
In fact, the djvu format is one of the most popular, as well as pdf, formats for storing scanned and e-books, magazines, documents, instructions. The picture quality is quite good and the file size is much smaller than pdf. The great thing about the format is that it is not just a picture. Simultaneously with the picture, a text layer is recorded, by which you can search or copy it from the book. Since the format is specific, you simply cannot open such a file - you need special programs for reading djvu files. Currently on the Internet you can find a whole lot of free programs that allow you to open djvu files. Here are the most common and well-known ones: WinDjViewer, DjvuReader, STDU Viewer.