The most common reason for this is that you are not using Adobe Acrobat Reader.
My files were created using Adobe Acrobat. Therefore, I strongly recommend using Adobe Acrobat Reader to view and print them. While other PDF reader software may work, they are more likely to have font and printing issues.
Adobe Acrobat Reader is free for both PC /Windows and Mac computers. You can download the most recent version here: https://get.adobe.com/reader/.