I am trying to wrap my head around the PDF file structure. There is a header, a body with objects, a cross-reference table and a trailer. In the official PDF reference from Adobe, section 3.4.4 about file trailer, we can read that:
The trailer of a PDF file enables an application reading the file to quickly find the cross-reference table and certain special objects. Applications should read a PDF file from its end.
This looks very inefficient to me. I can't show anything to users this way (not even the first page) before I load the whole file. Well, to be precise, I can - if my file is linearized. But that is optional and means some extra overhead both when writing and reading such file.
Instead of that whole linearization thing, it would be easier to just put the references in front of the body (followed by objects on page 1, page2, page 3... ). But people in Adobe probably had their reasons to put it after it. I just don't see them. So...
Why is the cross-reference table placed after the body?
I would agree with the two reasons already mentioned, but not because of hardware limitations "back in the day", but rather scale. It's easy to think an invoice with a couple of pages of text could be done better differently, but what about a book, or a PDF with 1,000 photos?
With the trailer at the end you can write images/text/fonts to the file as they are processed and then discard them from memory while simply storing the file offset of each object to be used to write the trailer.
If the trailer had to come first then you would have to read (or even generate in the case of an embedded font) all of these objects just to get their size so you could write out the trailer, then write all the objects to the file. So you would either be reading, sizing, discarding, then reading again, or trying to hold everything in ram until you could write them to the file.
Write speed and ram are still issues we contend with today when we're running in a docker container on a VM on shared hardware..