Yes, I suspect you may be correct in suspecting your USB dongle or keyboard from sending a power-on signal. I ran into a similar confounding problem where bluetooth keyboards and trackpads in a drawer were being pressed/activated and constantly waking my laptop up when it was connected to an external monitor!
I don't fully understand the task you were trying to complete, but Preview is not an image editor so I don't think it can do what Photoshop (or other image editors) can do. It can however combine multiple PDFs into a single document by drag-and-drop, and this is documented in Preview > Help.
I was trying to say something that is perhaps a bit more nuanced than "it's easy once you know how to do it" or even "it's easy to find once you know where to look" – specifically, the Mac UI was originally intended to be fairly discoverable by exploration (and relatively clearly documented), and although some of this has been lost due to iOS-ification, much of it still is:
1. Menu items such as Shut Down are easily discoverable (but in your case it didn't work, perhaps due to an external keyboard turning the system on.)
2. Export to PDF is very discoverable, but isn't universal; print to PDF is discoverable, and is documented in macOS Help, but is perhaps more subtle than it should be because it's in the Print... dialog. I suspect its location there is due to historical and implementation reasons rather than testing users who are unfamiliar with the system. "Preview" after all is named as a "print preview" application rather than a "PDF viewer" app.
3. Screenshots are discoverable by searching for "screenshot" in System Preferences or macOS Help. If you forget the keyboard shortcut it's in a fairly logical place: Keyboard > Shortcuts.
4. On macOS, the Help menu is contextual based on the current active application. Overall system help, titled "macOS Help", is located in the Finder's Help menu. This is perhaps a bit subtle for Windows users who may not know what the Finder is (it's like Windows' File Explorer) and might not realize that the Help menu is contextual. There are "macOS Help" entries for each of these tasks, but you need to know to look in "macOS Help".
While much of the macOS UI is still fairly logical, discoverable, documented, and consistent over many releases, that doesn't mean that a Windows (or Ubuntu) user will instantly know how to use it without learning some important design differences, or vice-versa. The good news is that once you learn them they're fairly consistent across the system.
An interesting point that you might be suggesting which I definitely agree with is that even after 40+ years of development, traditional desktop GUIs can still be greatly improved even without changing their fundamental design.
I don't fully understand the task you were trying to complete, but Preview is not an image editor so I don't think it can do what Photoshop (or other image editors) can do. It can however combine multiple PDFs into a single document by drag-and-drop, and this is documented in Preview > Help.
I was trying to say something that is perhaps a bit more nuanced than "it's easy once you know how to do it" or even "it's easy to find once you know where to look" – specifically, the Mac UI was originally intended to be fairly discoverable by exploration (and relatively clearly documented), and although some of this has been lost due to iOS-ification, much of it still is:
1. Menu items such as Shut Down are easily discoverable (but in your case it didn't work, perhaps due to an external keyboard turning the system on.)
2. Export to PDF is very discoverable, but isn't universal; print to PDF is discoverable, and is documented in macOS Help, but is perhaps more subtle than it should be because it's in the Print... dialog. I suspect its location there is due to historical and implementation reasons rather than testing users who are unfamiliar with the system. "Preview" after all is named as a "print preview" application rather than a "PDF viewer" app.
3. Screenshots are discoverable by searching for "screenshot" in System Preferences or macOS Help. If you forget the keyboard shortcut it's in a fairly logical place: Keyboard > Shortcuts.
4. On macOS, the Help menu is contextual based on the current active application. Overall system help, titled "macOS Help", is located in the Finder's Help menu. This is perhaps a bit subtle for Windows users who may not know what the Finder is (it's like Windows' File Explorer) and might not realize that the Help menu is contextual. There are "macOS Help" entries for each of these tasks, but you need to know to look in "macOS Help".
While much of the macOS UI is still fairly logical, discoverable, documented, and consistent over many releases, that doesn't mean that a Windows (or Ubuntu) user will instantly know how to use it without learning some important design differences, or vice-versa. The good news is that once you learn them they're fairly consistent across the system.
An interesting point that you might be suggesting which I definitely agree with is that even after 40+ years of development, traditional desktop GUIs can still be greatly improved even without changing their fundamental design.