ISO PDF have many versions , each version have different features and security .

PDF/A is an ISO-standardized version of the Portable Document Format (PDF) specialized for the digital preservation of electronic documents.

PDF/A differs from PDF by prohibiting features ill-suited to long-term archiving, such as font linking (as opposed to font embedding).

The ISO requirements for PDF/A file viewers include color management guidelines, support for embedded fonts, and a user interface for reading embedded annotations.

PDF/A-1 is based on the PDF Reference Version 1.4 from Adobe Systems Inc. (implemented in Adobe Acrobat 5 and later versions) and is defined by ISO 19005-1:2005, an ISO Standard that was published on October 1, 2005: Document Management – Electronic document file format for long term preservation – Part 1: Use of PDF 1.4 (PDF/A-1)

PDF/A-2 is based on ISO 32000-1 – PDF 1.7 and is defined by ISO 19005-2:2011, published on June 20, 2011 under the formal name Document management – Electronic document file format for long-term preservation – Part 2: Use of ISO 32000-1 (PDF/A-2).

PDF/A-3 is based on ISO 32000-1 – PDF 1.7 and is defined by ISO 19005-3:2012, published on October 15, 2012 under the formal name Document management — Electronic document file format for long-term preservation — Part 3: Use of ISO 32000-1 with support for embedded files (PDF/A-3)