In Windows 10, if you have multiple printers (including virtual printers such as Microsoft Print to PDF or Foxit PDF), and set your preferred one as the default, the default printer keeps changing automatically when you print to a non-default printer the next time.
The classic Devices and Printers Control Panel is generally used to set the default printer, but there is another setting in Windows 10 that overrides user’s preference.
Click Start, type printers, and select Printers & Scanners from the list. This opens the modern Printers and Scanners user interface.
This interface lets you select the printer and set it as default, or remove the device. Select your printer and click Set as default. Then, to prevent Windows from changing the default printer to the last used printer, disable the option When turned on, the default printer is the last used printer, listed under Let Windows manage my default printer. This option isn’t present in the Windows 8’s Devices app, so it’s new to Windows 10.
The classic Devices and Printers Control Panel doesn’t have this setting.
Check your Sync settings
If the above setting enables automatically after some time, or after a restart, then check your Sync settings. If "Other Windows settings" option is enabled in the Sync settings page, Windows would sync your printer, mouse, keyboard and other settings.
Open Settings (WinKey + i), click Accounts, Sync your settings
Disable the Other Windows settings toggle button.
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Ramesh Srinivasan founded Winhelponline.com back in 2005. He is passionate about Microsoft technologies and has a vast experience in the ITeS industry — delivering support for Microsoft's consumer products. He has been a Microsoft MVP [2003 to 2012] who contributes to various Windows support forums.