How do I tell writer to open at 160% not 84%? 84% is annoyingly small
So for me it’s like this, I open Writer set 160% and save the file.
The next time you open it, the document is at 160%.
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With me Windows 10 Home; Version 1909; 64-Bit | LibreOffice, Version: 6.3.3.2 (x64).
Open Writer (a new/empty writer/from scratch)
At the bottom right of the “Status Bar” (>View>Status Bar) you have a scale with a cursor:
moving the cursor will put your page a the desired scale, then close writer (do not save the empty doc, once it’s at the scale just close writer)
From now on All your new document will open at the desired scale, BUT if you have saved documents at a different scale they will open at the scale the document was saved.
For those old saved docs: you have to manually save them again with the desired scale, and you will need to change something in the doc itself, like add a space after a character to “save” the doc with the new scale, then remove that space and save it again.
If you only open your doc, change the scale and save your doc without changing something INSIDE the doc, the scale changes will not be taken in count.
Ubuntu-MATE 19.10 / LO 6.3.3.2 (AMDx64)
In addition to the above answers, if you want new documents to show up with a certain zoom factor, create/modify a template, save it with the desired factor, use it to create new documents. Unless you are as eager as ebot 
and would not mind setting the zoom every time.