Update by David M. Longo, Ph.D.
On August 1, 2022, the USPTO officially replaced its Public Patent Application Information Retrieval (Public PAIR) system with its new Patent Center system. Patent Center is accessible at: https://patentcenter.uspto.gov/
Users can still access publicly available patent and patent application file wrappers via Patent Center without a registered account in much the same way as via Public PAIR. With a registered account, users can access file wrappers that were previously available only via Private PAIR and can file new patent applications similar to EFS-Web.
According to the USPTO, Patent Center provides a single interface that allows users to both view and file patent applications—something previously available only via the separate interfaces of PAIR and EFS-Web. And, even though Public PAIR has been retired, Private PAIR is still an available interface for registered users who still desire the familiarity of the legacy interface.
The USPTO provides helpful information for users to get up to speed with Patent Center:
General Information: https://www.uspto.gov/patents/apply/patent-center
Frequently Asked Questions (FAQ): https://www.uspto.gov/patents/apply/patent-center/patent-center-faqs
Patent Center User Guide: https://www.uspto.gov/sites/default/files/documents/Patent_Center_User_Guide_Dec.pdf
Patent Center Quick Reference: https://www.uspto.gov/sites/default/files/documents/Patent_Center_public_doc_access.pdf
Retirement of Public PAIR: https://www.uspto.gov/patents/public-pair-be-retired