Basic Print Screen Trick

Salam Anak IT.
"Print Screen" key on my keyboard

How many of you treat the "Print Screen" key on your keyboard as a friend? Those who are doing assignments on "System's User Manual" will most probably familiar with this feature. Last time when I checked someone's system documentation from the technical college, I found out that there are people who always use the Print Screen key but are not doing it properly. What they always do is that simply paste the whole thing in a word processor that they are working on (such as MS Word) and then CROP the image... As results, there are "extra" irrelevant bitmap pixels on every edges of the image.
For example, I want to get the screenshot of one specific window. When I press "Print Screen" key alone, it will give me like the following image:

You do not have to worry if it is a little bit "weird" for you since I am using 2 monitor screens extended but yes, the "Print Screen" key did capture the whole screen... I only want to capture the PDF Reader's window with the "BRUNEI DARUSSALAM" wording in it... How am I going to do that?... Is that even possible?...
The "key" of making it possible is the "Alternate" key or also known as "Alt" key.
"Alt" key on my keyboard

Try to combine the "ALT" key with the "Print Screen" key and paste the clipboard item into any image processing applications (Adobe Photoshop, GIMP, MS Paint etc.) and see what will appear. Make sure that the targeted window is "active" in a way that it is "selected"? Hahaha lebih kurang miatu la...
The expected result

Try to experiment other combination of key as well such as CTRL key or even SHIFT key and examine the result for your own knowledge and skill.
That is it for today. If you have any doubts or questions to ask, let me know through this online form.
~ Terima kasih daun keladi, ada idea jumpa lagi ~
Sekian. Wassalam.