I have got a Surface Pro and very often I am using a second display, setting the Extend these displays [IT: estendi questi schermi] option in Settings [IT: Impostazioni]. In this way, I have two different screens that can be used to show different applications and so having more working space.

However, I find some anomalous display behaviors on the second screen connected to my Surface, like a small pdf preview on the right pane of File Explorer or an unexpected zoomed view in Photoshop Elements).

After some tests, I realized that these strange behaviors are due, in my understanding, to not so clean handling of Windows 10 of an external display (with a different resolution of the PC one) when it is set the extended screen behavior.

But let start from the beginning, that is how to set an external display to extend the PC one: obliviously if you know everything about this setting and you are only interested in the solution/workaround I found, you can jump to the next section of this post!

You can set the extension behavior of an external display, when connected to your PC, going into Settings -> Display [IT: Impostazioni -> Schermo] and then choosing to Extend these display [IT: Estendi questi schermi] option in the Multiple displays [IT: Più schermi] section:

You can go to display settings section even right-clicking in an empty desktop area and then choosing Display settings [IT: Impostazioni schermo] from the popup menu:

From that setting page, you can manage several displays. How much? At least two and this could be sufficient for your purposes, but take in mind that Windows 10 could handle even more displays and the only limit is due to how many graphic ports have your computer and how many monitors your graphics card does support. Anyway, you can always buy an additional graphics card, if there is enough space on your computer and you have enough slots! See this article for more information on several multiple displays handling.

Once selected one of the available displays you can both move it on the right/left just left-clicking on the rectangle (moving it and then release) and change definition/other options (like setting which one is to be considered the primary one).

Before finding the solution/workaround I read a post in the Microsoft forum and tried several suggestions that mentioned there because they could help in some cases … even though not so much in my case!
For example, if you are using Acrobat Reader DC as the default program for pdf file, you can try to set “Enable preview with PDF thumbnail in Windows Explorer” [IT: Abilita anteprima con miniatura PDF in Windows Explorer] option in its Preferences [IT: Preferenze] section:

I even tried to change the default program to open pdf files, setting it to Microsoft Edge, so to a Microsoft Windows embedded one. To do so, right-click on a pdf file -> Open with [IT: Apri con] -> Choose another app [IT: Scegli un’altra app]; then select a new app and set the Always use this app to open pdf files [IT: Usa sempre questa app per aprire i file pdf option]:

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How I solved, or better I found a workaround?

As I already said, I realized that the problem is a not clean handing of every single monitor by Windows 10 because of the relationship that is maintained among the setting of each of them!

So everything depends on how you set the resolution of each screen in relation to other ones, the percentage set to Modify text, app and other elements size [IT: Modifica la dimensione di testo, app ed altri elementi], and also on which one you set as the primary one!

In the following pictures I show you the result I obtained setting different resolution, percentage size settings and primary display selected.
Note that every time I did a change in displays settings, I did a PC restart to be always sure that they completely apply.

1. Primary display on Surface PC at 2736 x 1824 (recommended) resolution and screen size 150% (recommended);
   Secondary display on Philips monitor at 1920 x 1080 (recommended) resolution and screen size 100% (recommended)
   As you can see, on the external monitor there is the anomalous display behavior (e.g. pdf file preview very little).
   

   Primary display on Surface PC at 2736 x 1824 (recommended) resolution and screen size 150% (recommended);
   Secondary display on Philips monitor at 1920 x 1080 (recommended) resolution and screen size 100% (recommended)

2. Primary display on Surface PC at 1920 x 1200 resolution and screen size 150% (recommended);
   Secondary display on Philips monitor at 1920 x 1080 (recommended) resolution and screen size 100% (recommended)
   As you can see, on the external monitor there the anomalous display behavior persists but it is less evident (e.g. pdf file preview is little, but not more so much). Because of the screen size of the Surface, the set 1920 x 1200 resolution implies that two few millimeters unusable strips, one on the top and one on the bottom, while with the previous highest recommended 2736 x 1824 resolution all the screen was taken.
   

   Primary display on Surface PC at 1920 x 1200 resolution and screen size 150% (recommended);
   Secondary display on Philips monitor at 1920 x 1080 (recommended) resolution and screen size 100% (recommended)

   

   Settings for the internal Surface PC monitor

3. Secondary display on Surface PC at 1920 x 1200 resolution and screen size 200%; Primary display on Philips monitor at 1920 x 1080 (recommended) resolution and screen size 100% (recommended).
   As you can see, on the external display the view is perfect now while on the PC monitor the pdf preview is zoomed. This s due to the fact that on this secondary screen (now the embedded PC display) the percentage is 200% (recommended) while in the other is 100% (recommended).
   Again, because of the screen size of the Surface, the set 1920 x 1200 resolution implies that two few millimeters unusable strips, one on the top and one on the bottom, while with the previous highest recommended 2736 x 1824 resolution all the screen was taken.
   

   Secondary display on Surface PC at 1920 x 1200 resolution and screen size 200%; Primary display on Philips monitor at 1920 x 1080 (recommended) resolution and screen size 100% (recommended)

4. Secondary display on Surface PC at 1920 x 1200 resolution and screen size 100%; Primary display on Philips monitor at 1920 x 1080 (recommended) resolution and screen size 100% (recommended)
   As you can see, on the external monitor the view is perfect now and also on the PC monitor even though everything is a size too small to be used for working!!
   

   Secondary display on Surface PC at 1920 x 1200 resolution and screen size 100%; Primary display on Philips monitor at 1920 x 1080 (recommended) resolution and screen size 100% (recommended)

   

   Settings for external Philips monitor

5. Secondary display on Surface PC at 2736 x 1824 resolution and screen size 200%; Primary display on Philips monitor at 1920 x 1080 (recommended) resolution and screen size 100% (recommended)
   Therefore finally I preferred a solution similar to the number 3, but leaving the resolution of my Surface display to the recommended highest one (e.g. 2736 x 1824).
   The result is not completely the desirable one, because the size of the preview on the PC display is zoomed, … but is the best one among all combinations I tested!!!
   

   Secondary display on Surface PC at 2736 x 1824 resolution and screen size 200%; Primary display on Philips monitor at 1920 x 1080 (recommended) resolution and screen size 100% (recommended)

   The “strange” thing is that, if you disconnect the external display, maintaining the same settings for the Surface embedded one, the view of the pdf file preview becomes as expected, so no more zoomed!!!
   Therefore the setting of the external display (e. g. the percentage of dimensions), when connected to the PC, seems to influence the other embedded one, even though it should not!!