Generally, that is how they open. Can you give us some more info on what is actually happening when you open a pdf? Is it one in particular or is it all of them? What version of Reader?
As requested by~graffiti, to understand the issue in better manner it would be helpful if you describe it a little bit more?
I assume, is that you are referring to the file opens up from the same page where you closed last time?
For example- If you were reading a file at page30 and closed it. When you open the file next it opens up from the last view (page30) again.
If this is the case, then you can change this setting from- Edit>Preference>Document> Uncheck "Restore last view settings when reopening document "
Yes, that's right
I wonder how it works on mobile (Android)
reading a file at page30 and closed it.
When I open the file next it opens up from the last view (page30) again
at Mobile (Android)
This is a designed behavior of mobile application. You can not change the preferences setting in mobile app as you can on desktop application. By default it will open the pdf file from its last viewed page.