>From LaTeX, this is quite simple, there's a package for that - as for pretty much everything in the LaTeX world. Googling for just like 10 sec could have given you this great guide: https://webpages.tuni.fi/latex/pdfa-guide.pdf
Now, for just random PDFs, this is a bit more tricky, but you can do so with ghostscript. Now, this sadly doesn't have such a great guide, but something like this should do the trick, though that's only PDF/A-1 for all I can tell. If your contractor needs a different version, you'll have to adapt it: gs -dQUIET -dUseCIEColor -sProcessColorModel=DeviceCMYK -sDEVICE=pdfwrite -dPDFACompatibilityPolicy=1 -dCompressFonts=true -dSubsetFonts=true -sFONTPATH=/usr/share/fonts/ -o <file name of output> <file name of input> Now, one common thing that can happen is that you don't have the necessary fonts installed (I'm using the system-wide fonts path here, but you can also set any other path) so the result would look off. In that case, you could just convert the fonts into outlines, which will make text machine-unreadable and the file much bigger. For that, replavce "-dCompressFonts=true -dSubsetFonts=true -sFONTPATH=/usr/share/fonts/" with "-dNoOutputFonts". Since I'm not completely certain about ghostscripts defaults, you can also add "-dDownsampleMonoImages=false -dDownsampleGrayImages=false -dDownsampleColorImages=false" to make sure the images stay otherwise unchanged. For anything further, you'll have to research yourself as ghostscript is very complex but used by many people. Best Richard
