a) i open pdf24editor
b) i drop (to right) a pdf with, for example, 100 pages
c) i would like create a new pdf with only 1 page ( for example with the 58th page)
Open the PDF file. Locate the page you want to be extracted. Drag & Drop the page into the dark area. This creates a new document with the one page.
You can also create an empty document first. This can be done with the blank page icon in the toolbar. Then you can drag & drop the page into the blank document.