What “editing” a PDF really means here
A PDF is a finished page layout, not a word-processing file, so changing its existing words is difficult in any tool. Edit PDF takes the practical approach: it lets you add things on top of the pages — typed text, freehand drawing, highlights, shapes, arrows, underlines, strikethroughs, and images — which covers the everyday jobs of filling in a form that has no fields, adding a note, signing off a page, or marking up a draft.
It does not rewrite the text that is already in the document. If you need to change existing wording, the better route is PDF to Word, make the edit there, and save as PDF again.
The tools available
Text places a text box wherever you click, with a size you choose, a color, and bold or italic. Pen draws freehand lines and Highlighter draws a translucent band, both in a color and thickness of your choice. Rectangle, Line, and Arrow draw outlined shapes, Underline and Strikethrough mark text, and Image lets you place a picture such as a logo or a scanned signature. An Eraser removes marks you made, and Undo and Redo step through your changes.
Rectangles are outlines only, so they highlight an area rather than blank it out — to hide information on a page, use Delete Pages to remove it instead.
What happens when you export
When you export, everything you added is drawn permanently into the pages of a new PDF. That means other readers will show your additions exactly as placed, but they cannot be moved or edited afterwards — keep your original, or re-open the original in the editor if you need to change something. Text you add uses a standard Helvetica font, so characters that font cannot display (for example non-Latin scripts) are replaced with a question mark.
Editing happens in your browser, so the PDF is not uploaded.
Tips for neat results
Zoom in before placing text so it lines up with printed lines and fields. Use the same text size as the surrounding document, and pick a dark color for legibility. For formal signatures, the Sign PDF tool creates a proper signature block, and for review comments that other people can edit, Annotate PDF creates standard PDF annotations instead of flattened marks.