PDF/A-1a & PDF/X-1

There are several PDF formats to choose from, and some of them are meant for pre-press work, some others are for archival-quality documents. Others are regular PDFs.

LibreOffice can create PDF/A-1a (ISO 19005-1), which means Archive.

Scribus can work with pre-press ready files, so if you need to send to a large press machine, say printing thousands of copies of a brochure, or a magazine, then use one of the PDF/X (ISO 15930) variants:

PDF/X-1a -- all fonts need to be embedded and all images need to be CMYK or spot colors
PDF/X-3 -- Colour-managed, CMYK, Gray, RGB or spot colour data are supported.
PDF/X-4 -- Colour-managed, CMYK, Gray, RGB or spot colour data are supported.

But for the rest of us, PDF 1.3, 1.4, or 1.5 is the one to choose.