Hi, I know exactly how you feel. It took me ages to find out how to do this, even though I've done it before in Snow Leopard. Here's my best attempt to explain it:
1. Open the document you need to change. This is done by double-clicking on the file. The default program that pdf's open in is called "Preview" Its not a clever mode of opening it, just mac's equivalent of Adobe Acrobat.
2. At the top of the screen, there should be a toolbar that looks like this:
Preview File Edit View Go Tools Bookmarks Window Help
Click on the "File" button, and on the drop down menu that appears, click on "Export". This should be below "Duplicate" in the list.
3. A window will open that looks like a Save type window. The "Export As:" window is where you can name the file that you've exported. For examply, if I wanted to reduce a file called happy.pdf, I'd usually name the smaller file as happysmall (the .pdf bit is inserted automatically).
This window also lets you choose where you want to save the file to. Choose the folder you'd like.
4. Whilst still being in the "Export As:" window, at the bottom are two options. One says "Format" and lets you pick what file type you'd like to use. The other says "Quartz Filter". This is set as none. Click the button, and a drop down box will appear, giving you lots of options that I'm sure nobody ever uses. The second-to-last option is "Reduce File Size". Select this option.
5. Now click "Save", and the file you've created will be in the folder you specified. Done!
Note that because of the way this reduces the file size, if you have scanned files, they are likely to appear slightly fuzzy, and I would not recommend it. For typed documents, this method should be fine and I can't see any loss in the text quality.
I'm sure lots of people have tried o explain it on here below me, but try and follow this step by step and it should work.
C