These files are in Adobe's portable document format. If you do not have software to read pdf files, download Acrobat Reader--it's free. You can then either save pdf files to your hard drive and read them from there or install the included pdfViewer in your browser's plug-in folder and read the files online.
One of these days I hope to get around to webbing a straight html version of the Miscellany, but doing it in pdf is a lot easier. As you will see, I have broken the book up into about twelve separate pdf files so as to make reading it easier.