Admittedly the name 'wizard' implies do a whole lot more than one single task, but in fairness PDF Wizard does exactly what it claims to do - feed it a load of small PDFs and it merges them all into a single file. I can't think of many reasons why anyone would want to do this - perhaps creating an archive of dozens of small single page invoices? Anyway, it works well on the whole, despite an ultra-minimalist interface, shown below.

From the Store description:

PDF Wizard quickly and easily lets you merge multiple PDF files into a single PDF. Unlike many other apps, PDF Wizard does NOT upload your PDF files for processing--your PDF files stay safe and private on your device. Give it a try for free and see it work for you!

Here's PDF Wizard in action:

On the desktop, you can drag and drop PDFs - on the phone, use the 'Add PDF' control at the top. Select the ones to merge and then tap on 'Save' and give the merged file a name. And you're done - the merging is very quick, a second or so, perhaps showing that merging PDFs really isn't that hard at a binary level!

The file is saved where you specify, but you can also quickly preview it, at least using Edge - and you see here the boundary between two of the documents (with different page attributes) - it's clear that the merging process is just concatenating files, but hey, it works!

This is usually on offer in the Store. Currently it's down to £2, so isn't hard on the wallet if you need this functionality.