PDF/A is the ISO-standardised subset of PDF intended for long-term archiving, and converting to it makes a document self-contained: fonts embedded, colour profiles included, and everything that depends on the outside world removed. The point is that the file should render identically in twenty years on software nobody has written yet — which an ordinary PDF cannot promise, because it may rely on fonts installed locally or content fetched over a network. Courts, land registries, national archives and regulated submissions commonly require it for exactly this reason. The conversion runs on our server, since validating and rebuilding a document to the standard needs a document engine: the file streams through and the PDF/A streams back, with nothing stored, though it does leave your device. Expect the file to grow, because self-contained means carrying everything it needs.
What the standard forbids
PDF/A removes anything whose rendering could depend on the environment. JavaScript, embedded audio and video, external content references and encryption are all excluded, and transparency is restricted in the stricter conformance levels.
Fonts must be embedded — no exceptions, including the ones every reader is assumed to have — and colour must be defined by an embedded profile rather than left to the device. Together these make the file a closed system: everything needed to draw the page is inside it.
The conformance levels
PDF/A-1b is the baseline: the document will look right. PDF/A-1a adds structural tagging so the reading order and semantics are recorded, which is what makes a document accessible as well as durable.
Later parts, PDF/A-2 and PDF/A-3, permit newer PDF features such as transparency and, in the case of A-3, embedded source files. Which level you need is dictated by whoever is asking for it, and it is worth confirming rather than assuming — a submission rejected for the wrong conformance level is a slow way to find out.
Practical consequences
Files get larger. Embedding fonts that were previously assumed, and colour profiles that were previously implied, adds weight — sometimes substantially on a document using several typefaces.
Some content cannot be converted without changing it. Encrypted documents must be decrypted first, since PDF/A forbids encryption. Interactive elements and multimedia are dropped rather than preserved. And a scanned PDF converts happily to PDF/A while remaining unsearchable — the standard governs durability, not whether there is any text in the file.