I tried to download and install an older version of QGIS, but I don't understand how it works... I downloaded and opened this file: qgis-2.0.0.tar.bz2, but I don't see any program file here. I also tried to download the two .exe-files for this version, but I couldn't open them on my mac. What do I do wrong? I already have QGIS 2.18 installed.
The .tar.bz2 files are basically the source code for QGIS. You only really need to download these if you want to build QGIS yourself.
If you want to download an older version of QGIS for Mac, try the following link and scroll down until you find the QGIS archive:
Older versions of QGIS are only archived as Windows exe and source code. You can download older Windows versions from http://qgis.org/downloads/
To run them on a Mac, you need a Windows emulator, or a (free) virtual box which can contain a full version of Windows (supposed you have one, and register at Microsoft).
Alternatively, you could install a free Linux version inside a virtual box. For Debian, you can have QGIS version 1.4, 1.7.4, 2.4.0 and 2.14.10, and for Ubuntu you can have QGIS 2.0.1, 2.8.6 and 2.14.5 depending on the Debian or Ubuntu OS version.
All these are free of charge, and can use shared folders of the operating system.
Keep in mind that some plugins will not work with older QGIS versions, so you might catch and install older plugin versions manually if you need them. And current QGIS project files might corrupt if opened with older versions.