These statements on there aren't 100% correct:
* A portable file will not modify the Windows registry in any way.
* A portable app will not remember you personal preferences and settings. These may include recent or frequently accessed files and directories among other things.
* A portable app is unlikely to damage critical files on your system. There is always the chance that a malicious file might be bundled in the EXE file but that comes down to the app itself coming from an unreliable source. A portable app isn’t going to modify anything in the registry so there is less chance of things going wrong.
Sure a 100% true portable won't do that, but some actually will create registry data, and some will create AppData, and some both. That's why I always track the first time usage of portable software using Total Uninstall.
Also you can't really trust downloading the "portable" version of some software (lets use "portable" zipped Wise Disk Cleaner & Wise Uninstaller from the official homepage, and the PortableApps.com versions Wise Disk Cleaner Portable & Wise Uninstaller Portable as examples) because the only thing "portable" about them is there's no traditional setup file (well the PortableApps.com packaged versions sort of have a setup file, but it's just a pure SFX).
Those mentioned Wise software titles will create registry data (at least one CLSID key: HKEY_CLASSES_ROOT\CLSID\{E5B3AA32-A77C-4653-A7B1-1BF3EE35D531}) regardless of how you use the software, and they'll create AppData if you click the button in it titled Assistance - not to pick on Wise software because I actually like it, just using it as an example.
As for not remembering settings, some use .INI or .CFG, etc., files to remember your preferences - imagine if a portable web browser didn't remember preferences - well it would be the definition of completely useless.
A portable may not damage files on the system, however some may require the installation of some other software just to function which could cause system issues such as; Java, .NET Framework, a scripting language, etc. And again about the registry, some actually create registry data.
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A good source for finding portable software with user comments letting you know if it's not a 100% true portable is:
The Portable Freeware Collection: https://www.portablefreeware.com/
Of course you should already know about PortableApps.com,