Ask these questions to get a sense for whether your source is reliable:
Many databases allow or even encourage the formation of an account. These accounts allow you to do everything from saving specific resources to saving entire searches, which can be handy if it takes you a while to find the perfect combination of keywords and search limiters. They often allow you to email resources to yourself, which makes for a solid backup if something were to happen to your database account. For major projects, making an account is definitely something to take into consideration.
If you want to download and save your resources as .PDFs, make sure you keep them all in an easy-to-find folder with consistent labeling. Instead of keeping them as random strings of numbers, consider naming them by title, author.title, author.subject, or some other way that makes sense to you. When you are in the midst of writing, it's extremely unhelpful to have to spend a long period of time searching through randomly named documents for the one you need, only to have to do so again when you need a different one.