Having a built-in PDF viewer in Chrome is useful for short documents, but Chrome's insistence on using it even when you download a larger file and open it can be a nuisance. If you want the option of opening in a desktop viewer instead, here's how to disable it.
Swipe to close
chrome:plugins into the address bar, find the Chrome PDF viewer option and click Disable. Unfortunately, it's an all-or-nothing choice — there doesn't seem to be a way to tell Chrome to use its own viewer for direct addresses, but an external viewer for downloaded files.