The extension PUB is used by default by the desktop publishing software Microsoft Publisher. The Microsoft Publisher can be used to create print publications (eg newspapers), websites, and product presentations (eg flyers, brochures). The PUB format is a proprietary format that can only be read by very few applications.
PUB files can contain all types of text and fonts as well as graphic elements, including images and vector graphics. A PUB file is a print-ready file with all elements and colors ready for printing.
Unfortunately, PUB files can be opened almost exclusively with the Microsoft Publisher. Only Adobe InDesign can import PUB files using the PUB2ID plugin. If you do not want to spend so much money, you can also install the test version of the Microsoft Publisher and at least get the opportunity to view PUB files after the trial period has expired.
Alternatively, you can try to convert PUB files to PDF using various Internet converter services. For example, www.Pdfonline.Com/convert-pdf supports the conversion of PUB files. However, the success rate depends on the version of the MS Publisher that created the file.