Here is a web site you probably should bookmark: https://cloudconvert.com. You probably won't need to use it often but when you do have a need, it can be a lifesaver. Best of all, it is a free service if you only want to convert a few files. However, if you want to convert dozens of files, there is a modest charge.
CloudConvert converts almost any file from one format to almost any other format. It recognizes files in more than 200 different formats. I find it useful for converting PNG images into JPG format. It also can can turn a PDF file into a DOCX file, which you may open in Microsoft Word, LibreOffice, or Google Docs. It also can convert Macintosh Keynote presentation files into PowerPoint PPTX files. In fact, CloudConvert can convert about 200 different file formats into other file formats. It can also convert an entire web page into PDF, JPG, or PNG formats.
I find that CloudConvert does an excellent job of converting PDF files into editable .DOC or .DOCX files as its translation seems to be very accurate.
CloudConvert also allows you to extract audio from a video file. For example, I selected a .MOV file and converted it to .MP3 format. That gave me an audio-only .MP3 file. It also converts .MOV video files into .MP4 video files as well.
Did you download a video from the web, only to find it doesn't display in your computer's video player? CloudConvert probably can fix the problem for you.
CloudConvert will also convert files into more efficient versions of file formats. For instance, you can use the service to convert .JPG files (also known as .JPEG) into more compressed .JPG files. The result is reduced image size without losing the image quality. It is widely used in web publishing to reduce the image size maintaining the image quality.
CloudConvert is a cloud-based application. That is, the program runs from a web site. There is no software to install in your computer. It works equally well on Windows, Macintosh, Linux, Chromebooks, and even on mobile devices running Android or Apple iOS (iPhone, iPad, and iPod Touch).
For occasional use, CloudConvert doesn't charge. If you convert files smaller than 1 gigabyte and use less than 25 file conversions a day, CloudConvert's services are free of charge. For high volume users, CloudConvert does charge fees as described at https://cloudconvert.com/pricing. I suspect most in-home users will never encounter a need for fees.
CloudConvert is one of those useful web tools that you may not need immediately. But, the next time you say, "I can't open this file...," give it a try. CloudConvert may be able to convert that file you can't use into one you can.
CloudConvert is available at https://cloudconvert.com.