Troubleshooting

I uploaded a zip and Joomla did nothing useful

I cannot find the plugin

System > Plugins. Type filter = system. Search WebP Converter.
If the name looks like PLG_SYSTEM_WEBPCONVERTER, reinstall the plugin zip and clear cache.

Photos did not change

Plugin Status must be Enabled. Click Save.
Clear Joomla cache, then hard-refresh the public page.
Photos must live under the site Images folder (usually images/). Remote photos on other websites are not converted.
Check Exclusion Settings. Home page, menus, folders, or JPG exclusion can skip the file.
Turn Debug Mode on, reload once, read the message, then turn Debug off and Save.

Generate does nothing / no folders

Tick at least one folder.
Click Load more folders if the name is late in the alphabet.
Stay logged in as an administrator. Do not run Generate in a public (non-admin) window.

Delete did not remove files

Tick the original folder name, then delete. Check that the result lists real file paths.

Install warning about GD

Ask hosting to enable the PHP GD extension with WebP. Without it, conversion cannot run.

Phoca PDF (or other ?format=pdf links) show Failed to load PDF document

The plugin must leave PDF output untouched. From 2.6.1 it skips non-HTML responses automatically (PDF, JSON, feeds, and similar). You do not need to list every PDF URL.
Update the plugin zip, then System > Maintenance > Clear Cache, and open the PDF link again.
Disable for Components will not help: those pages still run as the article (for example com_content), not as a Phoca PDF component. Disable for URLs is also a poor fit, because the HTML page and the PDF share the same address.