Export, I am generating pdf files with pdf version 1.5. Is there a way to generate them with pdf version 1.4 instead?
You could down-convert the Mathematica pdf file using Ghostscript (I haven't tried that myself)
Run["gs -sDEVICE=pdfwrite -dNOPAUSE -dBATCH -dSAFER -dCompatibilityLevel=1.4 -sOutputFile=output.pdf input.pdf"]
There is currently an open case "CASE:1570212" about metadata in
Export options too. You could write to email@example.com requesting that feature.