With the pdfpages package, you can do more than just insert entire PDF files or individual pages. It also allows you to combine multiple pages from the original document onto a new page.
In the provided example, the nup option is used to combine four pages from the file named "Test" onto a single new page.
\includepdf[pages=-, nup=2x2]{Test}Besides nup, pdfpages offers other options like pages, which lets you specify which pages to include, and landscape, which changes the page orientation.