I seem to quite often find myself in a situation — home and at work — where I need to merge/combine a couple of PDF files. In macOS, this is quite easy to do as the OS has built-in PDF support. In Windows, you probably need to use Adobe PDF or a similar product. Even more tricky to achieve on a mobile device (I think).
So I hacked together PDF Blender — a small web app for merging pdf documents online - "on the fly". I know there are tons of these on the internet already, but I've never felt comfortable uploading my content to random servers.
PDF-Blender is powered by iText (a high-quality, well-maintained .NET + Java library for manipulating and creating PDF documents) under the AGPL license — which means that all source code, for all apps integrating it — needs to be disclosed.
To be clear, iText does all the "heavy lifting" here. The web app can be seen as the content server and a UI around it.
Feel free to use it. https://pdf-blender.com
I'm not storing any uploaded content on the server.