pdf files
     - require Adobe Acrobat (available below)
     - take at least 1 minute to download the whole document (can be longer)
     - are of very high quality
     - have adjustable view size, which is particularly useful
     - Acrobat allows you to print or save files easily

html files
     - download more rapidly, 1 page at a time
     - are of somewhat lower quality
     - have non-adjustable view size

Overall, pdf files are preferable, and it is certainly worth downloading Acrobat if possible.

You can download Adobe Acrobat free from HERE