Elsevier
Elsevier
You’re pushing it through one system that converts a PDF file into printer instructions, and then through another system that converts printer instructions into a PDF file. Each step probably has to make adjustments with the data it’s pushing through.
Without looking deeply into the systems involved, I have to assume it’s not a lossless process.
They maintain a high quality but not lossless.
As a trivial example, if you use the wrong paper size (like Letter instead of A4) then it might crop parts of the page or add borders or resize everything. Again I’ll admit, in 99% of cases it doesn’t matter, but it might matter if, say, an embedded picture was meant to be exactly to scale.
Those printer instructions are called Postscript and they’re the basis of PDF.
You are thinking that the printing process will rasterize the PDF and then essentially OCR/vector map it back. It’s (usually) not that complicated.