How can I access cache files in chrome in IoS?
If the PDF is still available online, you can access your browsing history in Google Chrome and find it there (if not already deleted from the history).

If the PDF is no longer available online, but online stored in the offline disk cache on your phone, you cannot access it from within Google Chrome, or from other apps on your iPhone.

You can however get to the cache by connecting the phone to a computer and use a program that allows you to explore the file system. That could be for example iExplorer from Macroplant or dr.fone Transfer from Wondershare.