PDF is a Portable Document Format, which is created by Adobe. Two intrinsic qualities make this Message Format the de-facto format for transmitting documents:
- Files that are generated in this format are more or less Immutable.
- It provides WYSIWYG across platforms.
PDF can (probably) be used to meet ALCOA Legible and Part 11.
The “best” tool to open PDF files, is Adobe Acrobat, or Adobe Acrobat Reader, for obvious reasons. However, nowadays there are many more tools that can open and render PDF files just fine, among which various Browsers like Chrome.