Unlike PDF, PDF/A forbids encryption. The usual purpose of encrypting documents is to prevent restricted material from being viewed by unauthorized personnel.
Encryption software is counterproductive for archiving documents because without the encryption key or algorithms or password the archived document cannot be viewed. The encryption might also be subjected to future alterations which will affect access to the document. PDF/A also requires the use of standards-based metadata.
PDF/A documents are entirely self-contained. It means that all content such as color information, text, images, fonts, and other elements must be embedded within the file. As already discussed, PDF/A documents should not contain any information from an external source.
These attributes give PDF/A files the advantage of reproducibility. When a PDF/A file is copied, it will still have entirely the same elements. PDF/A doesn’t allow room for subsequent modifications. Therefore, a PDF/A document created now would still have the same content in the future.
It provides guaranteed reproducibility and it fulfills the requirement that is needed to enable the accurate reproduction of content in the next 25 years or more. So anyone who opens the document in the next 25 years would be able to see the document unaltered.
The difference between PDF/A and PDF/X lies in the purpose of their creation. PDF/A was designed to keep files stored for a long time while PDF/X files operate based on the interchange of graphics or images. Though these are two specialized sub-versions of PDF they differ in key ways, in PDF/X files it is ensured that the images especially colors are accurately represented when shared.
It is ensured by creating restrictions on the types of color pallets that can be used, declaring the specific color pallet that was used and a statement in the document about what version of PDF/X is handling the image. Similar to PDF/A, PDF/X files do not permit active content within the file but this is to avoid any interference with the image.
The PDF/A is great for archiving but it is not without its own drawbacks or disadvantages. The evolution of technology definitely extends to PDF standards so perhaps there will be new additions or retractions to PDF/A capabilities and restrictions as seen with the newly published levels.