To enable the Save option in a PDF, you'll need to make sure the PDF you're working with allows for editing and saving. If the PDF is a read-only document, then the Save option won't be available.
Assuming the PDF is editable, you can enable the Save option by opening the PDF in a program like Adobe Acrobat or another PDF editing tool. From there, you can navigate to the File menu and select Save As or Save to save your changes. If the Save option is grayed out, you may need to check the document's security settings and make sure you have permission to save changes.
It's worth noting that PDFs can also be filled out and saved in some web browsers, such as Google Chrome. In this case, you can fill out the form fields as needed and then click the download or print button to save a copy of the filled out PDF to your computer.