Resizing and changing DPI of image easily

I've scanned several documents at 300DPI grayscale and saved them as TIFF (to not get any quality loss), and want to make a PDF file out of it all (I use a free application called Combine PDFs for this).

However, the 17 pages I've scanned turn into a 28 MByte PDF file, which is wayyy too big, but I'm not sure what to resize and how in order to get a more acceptable size.

I've already cropped each TIFF file using Photoshop Elements 2.0, meaning they'll be of slightly different dimensions. Next, should I change the resolution or the DPI, or both?
And what's a good tool for batch changing things like this? Photoshop elements has a "batch conversion" feature, but it doesn't allow me to change the size in percentage, just pixels.

Then there's Graphic converter 5.9 which came bundled with my Mac. I know it has some pretty powerful batch conversion features, but it's not the easiest application to use and frankly those features are quite confusing.

So what should I do?
I'm aiming for a PDF with good quality for screen viewing, but also acceptable for printing if this doesn't mean very large PDF files.