You can’t edit a PDF, at least if by that you mean you want a full-fledged formatting utility.
PDF is a display format. A PDF file is a collection of page images, not a text document.
Writer won’t be able to open it because a PDF is a graphics file. Draw will open it but you’ll be presented with a myriad of elementary graphical objects. Even what you take for a line is broken into homogeneous sequences of characters. Here homogeneous means same font face, size, weight, angle. Whenever one of these attributes if not the same, you get another sequence. Since the sequences are graphical objects, they are directly positioned at their “correct” location. You have nothing like alignment. This is taken care of by positioning the blocks where they’re supposed to lie to achieve the desired “alignment”.
But PDF rendering by Draw may not be the exact intended representation because the computer may not have the fonts installed. Or the PDF uses some formatting functions not handled by Draw.
All you can do is some sort of limited editing, just like you would edit a graphical file or a drawing, i.e. move a bit the objects, rotate them, change font properties individually. Needless to say this is not feasible on a 250-page document unless you only change one or two pages.
If editing is a real necessity for you (document contents is highly valuable), you can reconstruct a text document by copying lines or sequences and pasting them into Writer. You’ll have to cope with excess line breaks to rebuild paragraphs. You’ll have to define the required paragraph, character and page styles and apply them to text. Have the same layout is not guaranteed at all.
It’s up to you to decide if this worth so much trouble.
Edit your question to describe what you really want to “edit”. There might be other tracks to explore.
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