PDF is a well-known and widely-supported document format that is widely supported. It was first introduced in 1993 and was the initial version. If you look up the PDF document (and any other file that may have PDF information), You will find details about the PDF version that is included in the file.
The version of PDF released in 1993 is referred to as Version 1.0. Since then, numerous version updates have followed. Version 1 was a range of 1.0 to 1.7, but it has since become obsolete. It was addressed with a major overhaul to version 2.0 in the year 2017.
The process of checking your document for the PDF version can be quite handy, however. It is a format that is versatile, and, of course, e.g., PDF 1.2 documents are able to be opened using the latest version of Adobe PDF Reader. In some instances, more recent PDF versions are able to be opened and read in previous versions of the reader too. They might not contain or display properly displayed elements or items that depend on updated versions. When you look up the version that is PDF, it's possible to see whether your PDF file could trigger problems because the version does not fully support the PDF viewer you use.