A PDF or .pdf is a file format (like .doc or .jpg).
PDF stands for: Portable Document Format it’s a standardised format that can be read by most programs and web browsers.
Because of this standardised universal nature, you can rely on people being able to open and view any documents saved in this format. The same cannot be said for other formats that are closely linked to the programs that created them.
For example, if you share a document in ‘Publisher’ format, it would require your viewer to have the publisher app or program installed on thier device and since the highest percentage of your viewers are likely to be using a mobile device, it’s unlikely that they would have the ability to read your document.