Pages, text and fonts

The sheet itself, the words on it, and the lettering — carried inside the file so the page looks the same everywhere.

A page has a size, like a sheet of paper, and words are placed on it. The lettering — what a printer calls the typeface, the particular shape of the letters — is kept inside the document itself rather than fetched from whichever machine opens it.

That single decision is what makes a PDF worth sending. Give the file to a customer, a supplier or a printing works, and every one of them sees the page you laid out, in the lettering you chose, at the size you chose, whatever is installed at their end.

What it saves you

The page looks the same on every machine
Your reader does not need to own the lettering you used: it travels inside the file. Nothing is quietly swapped for something similar, so nothing shifts and nothing overflows.
The file stays small
Only the letters actually printed are kept. A one-page letter carries the few dozen characters it shows, not the several thousand the lettering contains — which is most of its weight.
It still opens in twenty years
Nothing in the file depends on a program that has to still exist. That is what makes a document worth keeping, and what an archive service asks for before it will take one.

See it done

Every one of these is a real PDF this library wrote, with the source that wrote it.

A minimal PDF

The smallest complete file: a page, and shapes drawn on it.