Applies to SharePoint Online / O365
If you have a requirement for to show search results from a specific Document library, here are the things you can do
Step 1.
Add a Search box
Step 2.
On the Search box > Properties > Send queries to a Custom search result > Provide the URL > https://XXXX.sharepoint.com/sites/KnowledgeCenter/SitePages/SearchResults.aspx?u={contexturl}
This is the new result page you will be creating in Step 3
Step 3.
Create a Search result page (any new page) and add a Search result webpart
Step 4.
Search Results Property > Search Criteria > Change Query > You can add the below in query text
To set the scope to a document library add "Path" and the path to exclude a path use "-Path"
Eg -
{searchboxquery} Path:https://XXXXsharepoint.com/sites/KnowledgeCenter/Lists/Posts/
-Path=https://XXXX.sharepoint.com/sites/KnowledgeCenter/Lists/Posts/StandardView.aspx
Step 5.
Run the "Test Query" and confirm the result set
All done, Hope this helps.