Most of these tools are obvious from their name. Three are not, because two of them overlap and one of them promises more than any software can deliver.
A scan that arrived as a PDF
This is a photograph wearing a document's clothing, and it is the most common source of an enormous file. Compress PDF is the right tool: it downsamples the pictures inside without disturbing the page structure, and it never re-renders your text.
A PDF you only want some pages from
Split PDF gives you every page as its own file, or a range as one new document if you type one. Organize PDF is the better choice when you know which pages you want and what order they should be in, because you can reorder and rotate at the same time.
A document that needs to become editable
Two answers, and the first one is usually better. Edit PDF changes the words in place, in the document's own typeface, without converting anything - use it when you need to fix a line rather than rewrite a report.
PDF to Word will get you something you can type in, but be clear about what it can and cannot do. A PDF stores positioned glyphs, not paragraphs, so the conversion rebuilds an approximation. Plain single-column pages come back well. Multi-column layouts, tables and heavy design will need tidying.
The same applies to PDF to Excel, which infers columns from the spacing of the printed page. A ruled, evenly spaced table converts well; a loose layout does not.
