That's the problem with all the pdf editors available out there. I guess adding stuff is much easier than editing existing stuff which you didn't add, and can be laid out in a gazillion ways, because PDF is a format for viewing and printing, not for editing.

I had a period when I worked with lots of documents. I could only find one fairly decent PDF editor which deserves to be called that way. I could use it to open any PDF and delete an existing element, or change text in an existing field. Sometimes things behaved funny, for example a text field can have each letter in a separate block, but I assume the tool did what it could within reasonable effort. It worked fine for me overall.

yes, that's why the description says "add text" not edit :)

there aren't existing packages that can edit existing PDF text in the browser, but I'll do it myself sometime soon

What I miss in free alternatives is the ability to redact. Not just by placing a black bar above, but such that the text also leaves the PDF. Shouldn't even need to reflow other text as mentioned in another comment, as things would otherwise stay the same.