I have to design something for an exhibition / trade show. As it's such a large print (230in x 100in) I made my file at half the size 115in x 50in. So when I save the pdf and zoom in to 200% because the pdf will have to be scaled x2, it look fairly good on screen but I am worried about how it will print as the images provided to me by my client are quite low dpi. So I was wondering is the PDF I'm looking at on my computer a good indicator as to how it will look when printed??
If you're only interested in scale...
100% zoom on your monitor is unlikely to be the same as 100% actual physical size. What you need to do is take your monitors resolution (PPI) in to account and calculate the actual zoom level you need. See Illustrator: View -> Actual Size smaller than life size for calculating the actual size zoom level in Illustrator or Photoshop. InDesign and Acrobat (not sure about Reader) do take your screen resolution in to account so will be more reliable, it's not guaranteed to be accurate though (you can set the resolution yourself in Acrobat's preferences if needs be). Other software may calculate size differently but are likely to be the same...
The easiest option, assuming you're not too worried about perfect accuracy, is to just grab a ruler (or a piece of paper or anything you know the size of) and adjust the zoom until the scale matches and note the zoom level.
(Then since your artwork is at half scale just double the "actual size" zoom level you noted.)
If you're interested in colors and print quality...
Not so much. Don't rely on an on-screen PDF to proof colors. Digital print shops can often do a PDF proof for you which will be more accurate but that is still nothing compared to an actual printed proof. With large format work like banners, especially if they're one-offs or very small runs, that's obviously not as practical but speak to the printers and see what they can do.
As for the image resolution, large format print work generally doesn't need to be anywhere near as high resolution as other print work (less than 100 PPI is often fine, it depends on the use though). Viewing your PDF at actual size on screen will give you an indication of how it'll look in terms of resolution but keep in mind that large format stuff is generally seen from far away and you're likely to see a small cropped piece on your screen so you may be fooling yourself thinking it's not good enough when in truth it is perfectly acceptable.
 See e.g. https://forums.adobe.com/thread/1871729