A PDF is not a document in the way a Word file is — it describes a finished page. That is why editing one feels awkward, and it explains which edits are easy and which are not.
Yes. PalPDF’s editor is free, has no account and adds no watermark. It runs in your browser rather than on a server, which is why there is no cost to pass on.
Usually because the editor used a substitute font. PalPDF reuses the font embedded in the document itself; where the document’s font has no glyph for a character you typed, it falls back to one that does and says so rather than printing an empty box.