A search for online PDF editors gave nothing but junk results. None of the editors I found listed - PDFescape, PDF Buddy, PDF Pro, PDFzorro, PDF Filler, sejda - provided a proper PDF editor.
Yes each editor provided a way to insert new content into a PDF and some even provided a way to overlay a blank area over the top of the existing content so as to add new content over the top of that (making a real mess of the actual PDF) but none offered proper edit capability for the PDF content and one of the editors even incorrectly claimed that editing the existing content was impossible (they obviously haven't used any of the downloadable PDF editors). Only one specified that their paid version would permit editing of the existing content (although it didn't say whether that would be done properly or via simply hiding the existing content and laying the new content over the top - presumably the former but given how limited these sites are, who really knows).
The way in which these sites can be useful is with regard to page manipulation. They all provide functionality for inserting and removing pages and rotating existing pages so they are useful for this level of editing if you don't have any software that can do this. They also allow you to add narrations etc to the PDF but then most PDF readers allow this.
The ability to insert text into the PDF does provide one useful function and that is that it allows you to complete any PDF form online using any of these editors.
Apart from the limitations of these so called editors not actually being able to edit the existing content of your PDF, there is also the matter of privacy. Each of these editors claim that they will discard your PDF when you are finished using their service but what documents are you prepared to trust them with. For some documents, being able to do all of the processing with the privacy of the document never leaving your own computer is essential and for those documents at least a downloadable program to do the edits is necessary.