Adobe Systems, Inc. created Postscript (PS), a printer language that allows rich output combining text and photos in high quality. Postscript files could be printed to any printer that was equipped to understand PS, so it became the de facto standard in the print industry. The problem is that Postscript files get HUGE. Any publication that is rich with photos and text has the problem of being readable and easily shared on a variety of computers that are not equipped with the software used to create it in the first place. You should expect people who create the complex layout documents and newsletters to use software like Adobe InDesign or Corel Ventura. If you do not own that software, you would be out of luck if you wanted to read it on your computer system and/or print it.
So Adobe created a system to compress the PS file to 10% of its original size, or less. A 10 megabyte file can be compressed to a 1 megabyte file. That is impressive, since it allows fancy work while minimizing the inconvenience to you. They called this compressed file a Portable Document Format file (PDF). It is small, it is portable across platforms. Then they created a FREE program (Adobe Acrobat Reader) that allows people to open the document and read or print it. So when PDF files are provided for you, this is good. They are MUCH smaller than the uncompressed documents, and you do not even need to know WHICH software was used to create the document. This is most important to you, especially if you are still using a dial-up modem.
The down side of this is that you cannot EDIT the file with Acrobat Reader. That is not a serious problem, normally, since you shouldn't really need to change it!