We use the Adobe PDF format for various documents, including free ebooks, because they are readable on nearly every computer and mobile device.
On Mac and Windows computers, you will need to install the free Adobe Acrobat Reader software before you can open a PDF file. Instructions on downloading and installing Adobe Acrobat Reader can be found at https://get.adobe.com/reader/
Once you've installed Adobe Acrobat Reader, simply double-click on the PDF file that you've saved to your computer and the PDF will open in Acrobat Reader.
Most Apple and Android mobile devices have built-in support to read PDF files. Usually just clicking on the link to a PDF file will open it on your device.