By "byteserving" do you mean writing binary data rather than text data? Yes, WebLogic supports that. I think that's a basic
J2ee requirement.
I haven't had problems with IE, but Netscape most definitely will give you grief if you don't supply a content-length with a PDF. It's quite well-documented.
Because you're running on a mainframe, you also need to be careful you know what codeset you're dealing with. A little slippage on ASCII isn't a big deal, but if you're creating PDF directives in EBCDIC you will create unreadable documents (which may cause the Acrobat Reader, hence the web browser) to hang or even crash. A lot of times you get "transparent" code translation when you bop about a z-System, but sometimes transparent is a liability, not an asset.
The secret of how to be miserable is to constantly expect things are going to happen the way that they are "supposed" to happen.
You can have faith, which carries the understanding that you may be disappointed. Then there's being a willfully-blind idiot, which virtually guarantees it.