It looks like this post contains a link that does not use a secure protocol: http://www.pedkat.pl/images/czasopisma/pk18/PK18_09.pdf.
_{HTTP is in use instead of HTTPS and no protocol redirection is in place. Be careful and do not enter sensitive information in that website as your data won't be encrypted.
It's also a good habit to always hover links before clicking them in order to see the actual link in the bottom-left corner of your browser.}