If you have iBooks on your iPad 1, you can save PDF's there. The current version of iBooks will not run on iOS 5.1.1, but if you've previously downloaded the app you can download the most recent compatible version to your iPad. I have iBooks on my iPad 1 and it works fine for saving and reading PDF files.
The same rules about downloading the latest compatible version of an app also apply to the Kindle app, which you can also use to save and view PDF's.