PDF is a Page Description Language. Every page is an autonomous set of objects. The content is stored in one or more streams. There is no such thing as a paragraph or a table etc in a PDF. It's just a sequence of lines, shapes and glyphs drawn on a page.
When you add content to a document using the
this content is converted into PDF syntax that is appended to the content stream of a page.
As soon as the page is full, this content stream and the corresponding page dictionary are written to the output stream and flushed.
Several objects, such as fonts, the cross-reference table, Form XObjects,... are kept into memory, because they can change during the document creation process.
In some cases you can release these objects early. For instance: there a "release template" method to write Form XObject to the output stream immediately. Image XObjects are always written immediately.