Each document type has a default app it will open with. When you double-click a document it opens in the default app. For example, double-click a JPEG and it opens in Preview. If you want to force a document to open in a different app there are various ways to do it.
The easiest is to right-click the document icon and choose an app from the "Open With" sub-menu. The document will open in the chosen app, but next time you will need to do this again.
To make the change permanent for a document (and allow the document to be opened with a simple double-click in future), click the document, choose Get Info from the File menu. In the dialog that appears, select the program you would prefer to use from the "Open with" menu. To change all documents of that type to that app, click "Change All" and confirm the change when asked.
You can also drag the icon for a document onto the icon for an application to force that app to open the document. The app icon will highlight if it can open the document.
There are several variations om this theme, too...
Here's an example: start dragging an icon from the desktop that you want to open in a program it would not open with my default, then, while still holding down the mouse or trackpad button, press command tab (use your right hand for the mouse and left for the keyboard) when you see the program you want to use to open the program just drag the icon onto it and release all keys and buttons. This might sound complicated, but when you get used to it, it is quite brilliant!
As an example, if I have a JPEG I want to edit in PixelMator Pro, I can't just double-click it because the default program for opening JPEGs is Preview (I know I could change the default, but most of the time I do want to use Preview). PixelMator Pro is always running on my computer, so it will be in the list displayed when I use the command tab trick. So I start dragging the icon, I press and hold command tab, I release tab, and drag the document icon to the app icon for PixelMator Pro, then release all keys and buttons.
You can also drag it to the icon in the Dock, but because I usually have the Dock hidden, I need another trick. Start dragging the icon, press command option D, the Dock appears, so drag the document icon to the program icon in the dock, release the mouse and press command option D again to hide the Dock. Alternatively, drag the icon to the area of the screen near where the Dock will appear (usually the bottom), when it does appear, drag the document icon to the program icon, and release the button.
I know these sound complicated, but they are easy after you have used them for a while, and they show how flexible the Mac OS is when used efficiently.
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