How to share only specific files with guests?

Agnieszka88 First of all check this external sharing documentations for more information how you can allow sharing files with the guests: 

1. External sharing overview 
2. Manage sharing settings for SharePoint and OneDrive in Microsoft 365 
3. Change the sharing settings for a site 
4. Collaborate with guests on a document 

Then you can use item level permissions for granting permissions to only specific documents/files: Set Specific User Restrictions on certain pages/files 

Make sure to remove guest user permissions from site/library level permissions if you have already granted those permissions.

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