PDF/X-1a explained

PDF/X-1a is the oldest and strictest of the print exchange standards. Its guiding idea is blind exchange: the printer should be able to output the file without asking you a single question, because everything ambiguous has already been removed.

That is achieved by forbidding a lot.

What it does not allow

• No RGB or Lab colour. Everything must already be CMYK or a named spot colour. There is no colour conversion left to interpret, and therefore no argument about who converted it.
• No live transparency. Drop shadows, blend modes and soft masks must be flattened into opaque artwork before export.
• No layers.
• No unembedded fonts. Every font must travel inside the file.
• No encryption, and no external references of any kind.

An output intent is required, so the file states which printing condition it was prepared for.

Why printers still ask for it

Because it removes every decision. If a shop runs an older RIP, or handles a high volume of files from customers of wildly varying skill, PDF/X-1a means nothing can arrive that needs interpreting. RGB cannot sneak through and get converted badly at the last moment.

The cost is that flattening is destructive. Once transparency is flattened it cannot be undone, and complex artwork can develop visible seams or stitching where flattened regions meet. You are trading editability and fidelity for certainty.

When to use something else

If your printer accepts it, PDF/X-4 is usually the better choice. It keeps transparency live, so nothing is destructively flattened, and still requires embedded fonts and an output intent. Use PDF/X-1a when the printer specifies it by name, which is a real and reasonable thing for them to do, not when you are guessing.

Checking a file

Conformance is not something you can confirm by looking at a PDF. Either run it through a validator, or upload it here and let Filecheck report which standard it actually conforms to and what would need to change.