I’m frequently asked how to import an already created bibliography (in Word or a pdf) into Zotero.
Perhaps surprisingly, there is no way to directly import text bibliographies into Zotero.
The best workaround, to be sure you import your full bibliography, is to add it item by item using isbn or doi, or by finding the citations in WorldCat or other databases and importing them to your local Zotero library.
However, if you have a large bibliography and double-checking for errors would be preferable to adding item by item, you could try processing your bibliography into a format Zotero can read with a third-party tool.
Some of these tools are free and hosted right on the web. Just be sure to closely review the final output.