Open the file C:\Program Files (x86)\LibreCAD\resources\library\templates\empty.dxf
Set your preferred settings in Current Drawing Preferences and save the file.
Then, next time you open a new file, the settings are there.
empty.dxf is the default template, loaded when you start up LibreCAD.
Also, when you create a frame inside empty.dxf, you will always start with that.
You can have more templates e.g. for different paper size or orientation and use New From Template to open them.
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This method does appear to ensure that the "Current drawing preferences" (most tediously otherwise, those on the dimensions tab) get loaded for new files.
But there does not currently seem to be any way of setting the default paper options for any printer?
All of my several physical and virtual printers are set at OS level (W10pro) to one or other metric paper sizes, but every one of them in LibreCad defaults to "letter", and even setting it for the first print in a current session does not help - it reverts to "letter" every time.
Is there any way to set LibreCad to respect a machine printing default of e.g. landscape A3?