PDF is by design an uneditable format, though there exist editors for it.
Due to the format definition, PDFs open in LO Draw as a set of graphic objects. Converting the text objects to a continuous text flow is tedious. You’d better open the file in a PDF viewer, select all and copy “as text” into the clipboard.
The clipboard can the be pasted into a blank Writer document. Check that you really paste text (if it doesn’t work at once, use “paste special” unformatted text). You get a document made of single-line paragraphs. You must then remove most of the paragraph marks to rebuild the human-relevant paragraphs. You must also restyle the text.
The “best way” to achieve your goal is to find and download a PDF editor adapted to your OS. If you can’t, proceed according to the workaround above.
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