Charles Hodge's Systematic Theology is available through archive.org as searchable pdfs. These scans are of a very high quality, it is the next thing to holding the original copy. (Remember, if you are using Zotero, you can link them to their bibliographic listing and then they will also be searched when you are looking for a resource.) He wrote his three-volume systematic theology in the 1870s and it is a supreme example of reformed theology.
Or, if you prefer to actually hold a book, check out CBD where you can get all three volumes in hardcover for a measly $24.99.