Adobe no longer support Flash on portable devices so that's one of the reasons there's no Flash support on the Vita
Secondly, the Vita is primarily a gaming device, not an internet device. The browser is functional for basic tasks.
Sony knows that most of us already have a decent browser on our mobile phones so why pay Adobe and so onexpansive licensing fees to enable usage of features which are not the Vita's primary functions
Neather FLASH or a PDF readers are needed, Sites designers are stuck in the past and will lose customers.
Googles new services run HTML5 video Players for PPV and use NON PDF for selling/giving away books that work inside the browser.
but because the PS vita IDs itself as a vita, the content the Browser gets will be second class always.
Even when HTML5 becomes finalised and a fixed standards are set. then released in non PC platforms.
It will still be a secondary device.
I still prefer m4a Audio books for realy stories to this version of a book.
Have you ever tried to create a 3rd party PDF reader to help you read PDF files with the help of some professional PDF toolkits? I am not so sure about whether it is the best solution to your problem. But I hope you success. Good luck.