I’ve spent months working on this document. When I loaded it today all the non-header fonts were set to Arial, the paragraph alignment’s been changed from justified to left, and the text boxes have been fragmenting into smaller text boxes, line by line or even word by word.
Also, every page has a white box in the background that’s caught by any attempt to select the text.
Am I in the wrong mode? Is there some way to salvage this so I don’t have to redo 175 pages of layout?
Apart from the Arial issue, it sounds as if you might have inadvertently loaded a pdf of your Draw document rather then the Draw document itself.
For 175 pages I would need an exceptional reason to use Draw rather than Writer. Even then, that reason would push me to Scribus (free) or inDesign (definitely not free).
This page might be a bit late for you, Preventing data disaster - The Document Foundation Wiki
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Some fonts cannot be embedded into pdf because of their license so a substitute font might be used if the exact font name is not found in the fonts on users machine. Sometimes the font name is written slightly differently in a pdf so font cannot be found on user’s computer so falls back to commonly used font. Best to avoid using fonts with such restrictive licensing. Search Google fonts for one with similar attributes
First, second and third priority: Backup.
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One directly on your system to have a safety, when you really do something wrong. Second on another disk/usb-drive on your desk. Third copy moved to another place you trust. If you also put this in folders with dates you are pretty well prepared.
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If @EarnestAl is right on the assumption you are editing a pdf, and there might be a draw-version you can use the search-function of your OS to search for the name of the file (without the .pdf). Start at the top folder and allow the system to search some time, if there is no immediate success. Attention: default windows hides extensions, so there may be no visual difference on first sight in explorer, if Draw is your default for pdf.
Just guessing: You started with a text document (.odt), exported to PDF, opened the PDF with LibreOffice. A PDF is not a text document anymore. It is a virtual print-out.
You can still open your original text document.
You can open the PDF with any PDF reader, for instance “Acrobat Reader” by Adobe. Do not open PDF with LibreOffice.