This prevents the Mac PDF Preview app from offering any scaling:
\usepackage[paperheight=842bp,paperwidth=595bp]{geometry}
I uncompressed the resulting PDFs with pdftk test.pdf output uc.pdf uncompress.
It turned out that \documentclass[a4paper]{article} leads to /MediaBox [0 0 595.28 841.89], which causes the Preview app to offer to scale down to 96% by default.
Next I tried \usepackage[paperheight=297mm,paperwidth=210mm]{geometry}, which leads to the exact same result.
Then I tried \usepackage[paperheight=296mm,paperwidth=209mm]{geometry}. This gives /MediaBox [0 0 592.44 839.06], but the Preview app still wants to scale down to 96%, even though the document is actually 1mm smaller than the A4 paper to be printed on!
I found only one format that leads to no scaling and that is /MediaBox [0 0 595 842], that is in LaTeX:
\usepackage[paperheight=842bp,paperwidth=595bp]{geometry}