If you are wanting an article or chapter that Purdue Libraries does not have access to because it's not available electronically or the physical copy is currently checked out then you can request the material through Interlibrary Loan (ILL). The scans MUST follow copyright - see the copyright tab for more information.
Request item through ILL
You will need to log into ILL to place your request
http://go.lib.purdue.edu/ill
Go to New Request
Choose PDF Article/Chapter request

Fill out the form and Submit Request

You will then see your request in the outstanding request box on the main page

You will receive an email from ill@purdue.edu when your items is available for download
Download File
You will log into your ILL account and go to View > Copies to Download

You will see your copy available to download and you can click View

When the chapter opens in your browser, select download and choose the location and name you want to save the file
Next Steps
Follow the instructions on how to Add Scanned PDFs to Your Reading List