FIX:
On download icon, right click and Open link on new tab
then change link in address, for example:
from:
https://odoocdn.com/web/content/slide.slide/1292/binary_content?download=true

to:
https://odoo.com/web/content/slide.slide/1292/binary_content?download=true

Hit Enter and the file will be downloaded!
Do the same with other documentation files.

EXTRA STEP:
After you download the file you will find that it doesn't have a file extension, no worries just add .pdf at the end of file name and it will open in your pdf application/program like any other pdf file.

Thank you.