You can zoom all pages of a PDF for all users using the zoom tool. Zooming is saved to the PDF file, so it affects all pages of that PDF and all users who are viewing that PDF.
Tap the Zoom button in PDF chooser
First tap the screen once to bring up the toolbar, then tap the file name in the center of the toolbar to open the PDF chooser. Finally, tap the Zoom button to zoom the checked file.
Pinch to zoom
Place two fingers on the screen and "pinch" them apart to zoom in, or together to zoom out. Once you are satisfied, tap the Done button at the top right.
Zooming a PDF changes the zoom for anyone else viewing the same file on a device that has the same screen aspect ratio. All iPads have the same aspect ratio, but Android devices have a different ratio. If you adjust the zooming on an iPad, all other iPads will see the same zooming, but Android devices with a different aspect ratio won't see your changes.
Because the aspect ratio is different when the toolbar is visible, your zooming will look different with the toolbar than without the toolbar.