{pdf-a} is about the PDF/A file format, an ISO-standardized version of PDF ({pdf}) specialized for the digital preservation of electronic documents (the "A" stands for "Archive").
PDF/A is an ISO-standardized version of the Portable Document Format (PDF), specialized for the digital preservation of electronic documents (the "A" stands for "Archive").
PDF/A differs from PDF by prohibiting features ill-suited for long-term archiving, such as font linking (as opposed to font embedding).
For more details, visit the Wikipedia page for PDF/A.
If your question is about PDF in general, use the broader pdf tag.
