Can't save downloaded pdf documents

I want to download some documents from the medical library, but when I do, the document opens allowing me to read it, but won't allow me to save. The only options in the upper left hand corner is print icon and an envelope icon (for emailing the citation). Occasionally I'll open a document that shows a small floppy icon, allowing me to save the document onto my computer. But many times, that icon is missing, and the only thing
I can do is read the document.

Can you explain why this happens, and how to prevent it? I'm working on my dissertation and need to be able to save research articles that I find.

Thanks.