If I have uninstalled Adobe Reader, but I can still open PDFs in Chrome, what application am I using?
Chrome itself is the application that is viewing the PDFs.
Windows 10 Edge can also open PDFs directly. Please note, IE is an older, separate application from Edge. If your users start in IE, they will not be able to open PDFs directly in IE unless they have Adobe plugin installed.
Firefox is able to view PDFs without a plugin : https://support.mozilla.org/en-US/kb/view-pdf-files-firefox .
Some websites (bank websites in my experience) will force a download rather than allowing it to be opened in the same browser. In Firefox, I was able to sometimes change the behavior by changing the MIME settings, but it didn't work consistently. https://support.mozilla.org/en-US/kb/change-firefox-behavior-when-open-file
From memory, Google Chrome uses a built in PDF viewer which was written in JavaScript I believe C++, thanks to the helpful reminder in the comments :)
This comes bundled with Chrome by default.
For me, I can view some of the related JavaScript here:
chrome-extension://mhjfbmdgcfjbbpaeojofohoefgiehjai/pdf_viewer.js
You can view some related code by opening the Chrome DevTools on the PDF Viewer.
Firefox uses another similar approach, again, written in JavaScript, although their documentation is more publicly available & from memory, is encouraged for others to use to display PDFs on their website.
Read more about Firefox's implementation here: PDF.js by Mozilla
As the chrome is derived from the chromium open-source project, it also uses the same embedded pdf-viewer that is shipped with Chromium Browser. So when a pdf file is encountered, chrome tries to open it using this pdf viewer.
If you open the
chrome://system
you would notice that it shows the open tabs and running extentions, but it does not shows any info about pdf viewer it is using here.
But when I open this
chrome://local-state
chrome shows multiple pdf plugin components including Adobe Reader and Chromium PDF reader. It might differ in your system.
The Readme docs are available for the chromium project, but I could not find the docs for pdf viewer. You can read it here