It is recommended that users with this operating system try using Firefox instead to solve the problem. This will lead to you being unable to open PDFs and a number of other problems. If you are running OS 10.5 x, as many are, you will find that the later versions of Safari, for example, are non-compatible with your operating system. Solution: This issue depends on your operating system. You can then make annotations and markups within your document. An outdated browser may be the issue, in which case you may need to update your browser to resolve the issue. You can usually run a regular software update, but other times you may be required to update manually through the menu. Not all browsers update automatically, and it could be that yours needs a manual update. Solution: This is one of the most common problems that can prevent you from being able to read a PDF. Here are the three main reasons why you may have a problem reading a PDF on Mac and what you can do to solve the problem. This is due to a variety of reasons, but luckily this problem is usually quite an easy one to solve in just a matter of minutes. Many people experience problems when trying to open or read PDFs on Mac. Part 2: Troubleshooting on Opening and Reading PDFs in macOS