I highly recommend SumatraPDF or MuPDF if you're after something a bit more. You may or may not need an add-on or extension to do it, but it's pretty handy to have one open automatically when you click a PDF link online. Most web browsers, like both Chrome and Firefox, can open PDFs themselves.

It's completely fine to use, but I find it to be a somewhat bloated program with lots of features that you may never need or want to use. Adobe created the PDF standard and its program is certainly the most popular free PDF reader out there. Most people head right to Adobe Acrobat Reader when they need to open a PDF. PDF files always look identical on any device or operating system. The reason PDF is so widely popular is that it can preserve original document formatting. The Portable Document Format (PDF) is a universal file format that comprises characteristics of both text documents and graphic images which makes it one of the most commonly used file types today. What is a PDF (Portable Document Format) file?

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