Apple's latest iOS update fixes the PDF bug that allows the latest jailbreak.
If you don't plan on jailbreaking your iPhone, update your iPhone soon. As Apple notes, this bug means that merely "(v)iewing a maliciously crafted PDF file may lead to an unexpected application termination or arbitrary code execution."
If you have jailbroken your iPhone, do apply the patch that fixes this bug (but not, presumably the update from Apple).
Apple has a note about 4.3.4 here.
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