Adobe Acrobat Reader is
required to view PDF files.
HTTP stands for Hyper Text Transport Protocol, which is just a fancy way of saying it's a protocol or language for information to be passed back and forth over the Internet between web servers and clients (you).
The letter S, which makes the difference between HTTP and HTTPS stands for Secure.
If you visit a website or webpage, and look at the address in the web browser, it will likely begin with the following: http://. This means that the website is talking to your browser is unsecure. In other words, it is possible for someone to eavesdrop on your computer's conversation with the website. If you fill out a form on the website, someone could see the information you send to that site.
This is why you never enter your credit card number or any username or passwords in an http website!
If the web address begins with https:// that means your computer is communicating with the website in a secure code that is very difficult, if not impossible for someone to eavesdrop on.
If a website asks you to enter private information, you should always automatically look to see if the address begins with https://. If it doesn't, do not enter sensitive information such as a credit card number or banking website login information!