There is no single "update from master" command. Come to think of it, there isn't even a link from a document "back" to the original (let's just call it a "template" per Adobe nomenclature, rather than the suspiciously Quark-sounding 'master').
A script can solve this only if either (a) you always use the same single template -- then it can be hardcoded into this hypothetical script --, or (b) you are prepared to do some extra work by storing the name of the template file somewhere inside derived documents. Option (c), at run-time asking "Where is my Master?", is not applicable because it would require human intervention.
What sort of updates are you thinking of? Since there is not a single "update all" command, you get to choose from Cell Styles, Character Styles, Object Styles, Paragraph Styles, Stroke Styles, Table Styles, and TOC Styles.
Anything beyond this list must be done 'manually', i.e., if you think you can change the page size and margins in a template and expect this to be 'copied' automatically across your hard drive, you'll have to write a specific script for this job alone.
... export each into PDF without any human interaction.
That's putting a lot of trust into an automated system such as you are dreaming of. Even the fairly standard job of "import paragraph styles" out of another document can do all sorts of harm to a random text: unsupported text styles (not every font has bold or italics), unsupported glyphs, other word and letter spacing that may leave part overset ... Even with my personal & human interaction I'm always very careful when I absolutely have to do this.
I think master is the worng terminology… Sounds more like using a book… And picking a style source and syncing options…? There is a synchronizeMasterPage in there?