PDF/A is an international ISO standard for archiving PDFs. The standard requests strict compliance to its set of rules (like: "embed all fonts", "don't use transparencies", "don't use JavaScript", "no encryption",...).
There are a lot of PDFs out there which claim to be PDF/A, but fail a real smoke test. That claim is just a tag in the file's metadata. That tag can make f.e. Acrobat Reader display a special hint when rendering it.
A check for real compliancy requires some rather expansive commercial "preflight" software. Currently I'm not aware of any Free utility to do that job. See also here for some test results: Isartor testsuite
You can use Ghostscript to (try to) convert PDF to PDF/A. How to do this is documented here.
But note: this document was updated only very recently [*]. Previous versions of Ghostscript's Ps2pdf.htm did mis-lead users to run a command that created PDFs claiming to be PDF/A but which failed real smoke tests.
How to convert PDF to PDF/A with Ghostscript:
Here is a commandline:
gswin32c ^
-dPDFA ^
-dNOOUTERSAVE ^
-dUseCIEColor ^
-sProcessColorModel=DeviceCMYK ^
-sDEVICE=pdfwrite ^
-o output_pdfa.pdf ^
-dPDFACompatibilityPolicy=1 ^
PDFA_def.ps ^
input.pdf
[*] Note: The problem lays with the parameter PDFA_def.ps. This is a file you need to edit to suite your needs. Ghostscript ships with a sample of it in its /lib subdirectory. This sample will not work as-is without you editing it. How to edit is inside the sample's comments.