The default PDF viewer is something that you set up on the computer. Each user can change the default.
You can set up the document to open in single page continuous mode by opening it in Acrobat and selecting: File > Properties > Initial View > Page Layout > Single Page Continuous
Not all PDF viewers (e.g., Chrome's pdfium) will respect this setting, but Acrobat and Reader do.
You can also set a default for Acrobat/Reader by setting either of the followingf user preferences:
Preferences > Page Display > Default Layout and Zoom > Page Layout
- OR -
Preferences > Accessibility > Override Page Display > Always use Page Layout Style
The latter overrides the former.