You do not have to use Adobe to open PDF files. Foxit is a very good alternative PDF reader. Have been using it for quite a while, and you can set it as default reader.
Sometimes in Firefox, it will open PDFs in its own reader. Up the top right corner where the menu icons are, there is a print option indicated by the printer symbol. Next to that is a download option, and clicking on that gives you the option to save the file to your computer, or opening it in your PDF reader, such as Adobe or Foxit. I usually open the file in Foxit and print it from there as I know it works. Printing from Firefox does not always work, as I understand it is a very basic reader and may not work with the way some PDFs are set up.
Ian C