Oh Denis may have been wrong with his own file, but I had this problem and I can guarantee that that was with a file compiled with pdftex.

I had  to use Acrobat to 'fix' the pdf file and make it acceptable for this archive system, which is mandatory for all phd thesis in France.

As this problem surfaced again on the GUTenberg mailing list, I decided to ask.

Here is my attempts width the attached test.tex file :

- without the use of the pdfx package, the validator returns 

  Message : No document catalog dictionary

- with the use of the pdfx package, the validator returns

  Message : Lexical error

  which is annoying and may indicate some kind of error 

  (but is it low level in pdftex or rather in the pdfx package ?)

- after using Acrobat and saving test.pdf as pdf/a-1b, the validator is accepting it.