We have full-text access to part of JSTOR, (select Caltech Library to sign-in with Access credentials) but not the entire collection. If the article or chapter you're interested in does not have a "Download PDF" link, please request it through DocuServe, our Interlibrary Loan and document delivery service.
You can also limit your search to items you have access to: scroll down to the bottom of the Refine Results sectionand select 'Content I can access' under Access Type.