PDF/A, where the ‘A’ stands for Archive, is a subset of the PDF standard. The key goal for this version of the format is that the file should be completely self-contained. This means all elements, including fonts, have to be embedded within the file.
PDF/X is designed primarily for document eXchange and is formalised into two ISO standards, 15929 and15930, though 15930 is the only current one. There have been eight revisions to this standard so far, with the latest based on PDF 1.6. It’s likely there will be further releases of this standard, designed to take advantage of updates to PDF itself.
Three important improvements to PDF have been electronic signing, form filling, and the ability to access and edit the contents of a PDF file directly.
Read more about PDF in the Modern World.