Putting two PDFs together sounds like putting one file after another. Underneath, it is closer to building a new document from the pages both files already contain. Those pages can have different sizes, orientations, fonts, images, links, and internal structures; they do not need to be flattened into pictures or resized to match each other. That is why I treat the page as the thing being moved, not the screenshot you see on screen. You can bring several PDFs in, arrange their pages in the order you want, rotate or remove what you do not need, and then save the result as one new PDF. An A4 page stays A4, a landscape page stays landscape, and the source files are left alone. The whole operation happens in the browser. Your PDFs are opened on your device, combined there, and the new file is created when you save it. There is more on how PDF merging actually works in Guides.