PDFStitcher is written in Python and bundled into executables for Windows and Mac. It does not need to be installed; just download and run.
After downloading pdfstitcher.exe, you can put it wherever you want - your desktop, your user folder, a cloud drive, etc. It is a self-contained executable so you just double click to run.
If you’re using a Mac, PDFStitcher will only run on OS X Catalina (10.15) or newer. Additionally, you may get a warning that you are trying to open an app from an unidentified developer. Please see this article for instructions on how to override this warning.
After downloading you can double click the .dmg and drag and drop pdfstitcher.app to your Applications folder (or again, it can really run from anywhere you want to put it). This is a new process as of v0.4 - older versions distribute the .app as a zip file.
PDFStitcher is available on Flathub.
PDFStitcher can also be installed from PyPi for Python 3.3-3.9 with the command:
pip install pdfstitcher
This will install pdfstitcher as a module and a script that can be launched with:
python -m pdfstitcher
Note that this will not work with Python 3.10+ until wxPython is updated.