Open SQL Server file
A MDF is a primary database file, ndf is a non-primary database file, ldf is a log database file. Together, these objects create a database in SQL Server. These files can be attached or detached from the SQL Server Instance.

Hierarchy is: SQL Server Instance -> SQL Server Databases -> SQL Server Schema -> SQL Server Tables.

The most common way to attach and detach a database is to use SSMS. (SQL Server Management Studios) SSMS has recently been placed as a standalone install, you could download SSMS on a Windows machine and then follow this KDB to attach the database to your Instance. I would recommend using caution when attaching unknown databases to your instance. You will also need to attach the log file and any other files that were included.

Once it has been attached, you may be able to view the objects inside it. This will depend on whether the database is encrypted with Transparent Data Encryption, what user permissions are set to, and other factors. 

Objects include tables (this has data inside it which you are probably looking for), stored procedures (programmed scripts that can be executed and run), views (queries that look at tables to retrieve a set of data in a specific fashion) and may other things.