In the latest version of Gnome 3, a subtle change was brought to Nautilus, the file manager, to automatically extract the content of a ZIP file in a new folder when it is double-clicked on instead of open an application to do so. The problem is that Microsoft Office files (.docx, .xlsx, .pptx, etc.) are ZIP files. This means that double-clicking on a .pptx file extracts its content instead of opening the file in, say, LibreOffice Impress!
Fortunately, it is quite easy to make Nautilus become sane again. Open Preferences and in the Behavior tab, unselect “Extract the files on open” in the Compressed Files section.