How do I access my SD card through a Windows USB connection? I'm trying to upload a simple PDF file to my Droid X Android 2.3 phone. I connected the cable and uploaded the file but I don't know which folder to put the file into. Android's PDF applications aren't helping at all and they all seem to look for files in SD. How do I access SD contents from Windows? How do I know if I have an SD card in my phone?
Most Android file managers and dialogs start you off in the SD card; if for some reason yours isn't, it's located at /sdcard .  This is also the directory that is mounted when you hook up your phone to Windows via USB.  

Just to make sure here, you aren't trying to access the PDFs while the phone is still mounted in Windows?  You won't be able to, if that's the case.  

If you want to look around more on your phone to find out where the PDFs are, you need to use a file manager on your phone. Most phones come with one preinstalled, simply called "Files" in the app drawer. Usually they open directly to the SD card directory (which is /sdcard in the root file structure).

If you don't have a file browser already installed, try ES File Explorer from the market, it's quite good and has boatloads of features. It also starts off browsing in the sdcard by default, and if you have root you can explore the entire file system, not just /sdcard .  It also has a "Search" function which may help you find your PDF files by filename.