What is PDF/A and when do you need it?
A court portal, a university repository or a public tender rejects your file: “the document must be PDF/A”. It is still a PDF, with a few rules that guarantee it will look the same decades from now.
What makes PDF/A different
PDF/A (ISO 19005) is a subset of PDF for long-term archiving. Everything needed to display the document must be inside the file: all fonts embedded, colors tied to an embedded color profile, and metadata that declares the standard. Anything that depends on the outside or can change is forbidden: encryption, JavaScript, audio and video, content loaded from links.
1b, 2b, 3b: which one?
PDF/A-1b (2005) is based on PDF 1.4 and does not allow transparency. PDF/A-2b (2011) allows transparency, layers and JPEG 2000 and is what most institutions ask for today. PDF/A-3b is like 2b but lets you attach any file, such as the XML of an electronic invoice. The “b” means basic conformance: the document looks right; the stricter “a” level also requires a tagged structure for accessibility.
If you are not told otherwise, choose PDF/A-2b.
How to convert
PDF to PDF/A rewrites the document with Ghostscript: it embeds the fonts, adds the sRGB profile and writes the metadata, without recompressing your images. Unlock protected files first with Unlock PDF, since PDF/A cannot be encrypted.
How to check it
The converted file declares its PDF/A level. For an official check, institutions use validators such as veraPDF, which is free; if one reports a problem, the most common causes are damaged fonts in the original or transparency in a PDF/A-1 conversion.