There’s no specific Merge command or function, but there is a clumsy workaround. It may be more trouble than it’s worth, though.
Take whichever PDF you want to be the first to appear in your new combined PDF and make a copy of it. Give it the title of the new, combined PDF you want to create.
Open this in LibreOffice. It will open in Draw.
Go to Insert > Page (or right-click on the last page of the PDF and select New Page) and do this as many times as necessary to create the number of new pages you will need to add your other PDFs. (I don’t see any way to bulk add new pages.)
Click on the first blank page in the series. You will see that there are two columns. All the pages of your document appear in the left-hand column, the Pages pane. The one you are currently working on also appears in the right-hand column, which is called the Workspace.
If the second PDF you want to add to the first is only one page, you can drag in into the blank page in the Workspace. You will probably have to adjust the position of these additions, as they tend to paste a little bit askew.
However, if it’s a multi-page PDF, you will have to open it in another program such as Adobe Reader and copy and paste the contents of each individual page. Because when you drag and drop into Draw, only the first page gets pulled in. And there will probably be some re-formatting you need to do.
Repeat for all the other PDF’s you want to add.
Save and then File > Export as PDF.