PDF/A is a subset of PDF and is used for archiving information. In order to preserve the information in the file and ensure that even after long periods of storage, the content can still be displayed as it is, but when it comes to comparing it to PDF, there are many obvious differences between the two. PDF/A sets stricter standards than those used by PDF.
The first major difference between PDF and PDF/A is the latter’s restrictions on certain types of content. You cannot embed audio, video, and executable files in PDF/A, because PDF viewers cannot open them individually, and there is no way to be sure that appropriate software for them will still be available in the future.
Images are allowed to be included in PDF/A documents as long as they are embedded, along with the fonts used to render the document. This ensures that these resources are always available. In general, PDF/A does not allow a file to refer to any external resource, because there is no way to tell if that resource exists. If the referenced external resource cannot be found, it may cause the documentation to display incorrectly.
Finally, PDF/A files cannot be encrypted for the exact same reason above. Encryption is a method used by preventing any unauthorized person from viewing any restricted material. PDF allows different types of encryption and restrictions. In PDF file format, you can increase the security of PDF documents by setting user and owner passwords. Additional safeguards include setting restrictions on printing and copying PDF content.