PDF is a broad document format that can contain interactive forms, multimedia, encryption, attachments and many other features. PDF/A is a constrained PDF family intended to preserve a static visual representation over time.
A PDF can open perfectly today and still fail a particular PDF/A conformance check. The difference is not simply the filename extension.
Choose ordinary PDF for active workflows
Use regular PDF when passwords, interactive behavior, frequent editing or workflow-specific features matter. It is the practical choice for most everyday sharing and review.
Choose PDF/A for preservation requirements
Use PDF/A when an archive, customer, regulator or records policy requests a named conformance target. Fonts and color information may need to be self-contained, while encryption and certain dynamic features are incompatible with archival goals.
Do not confuse conversion with preservation
After conversion, validate against the required PDF/A part and conformance level. Also preserve metadata, naming, checksums, access controls and migration procedures. Long-term preservation is a system, not a one-click file setting.