There are two major factors to consider in whether to accept a digitally signed file: the macro security level setting in Access, and what actions you will take when you open the file.
At the Medium macro security level, when you open a digitally signed file for the first time, either you can open that file right away (by clicking Open), or you can permanently add its signer to your list of trusted publishers. If you add its signer as a trusted publisher, Access will treat any other files with that same digital signature as coming from a trusted entity.
At the High macro security level, things are buttoned down a bit more tightly. To be opened, any file must be digitally signed (no exceptions), and you must accept the digital signature and permanently add its signer to your list of trusted publishers. As with the Medium macro security level, any other files with that same digital signature will then be treated as coming from a trusted entity.
In Access, unlike most other Office programs, it simply isn't possible to open a file that has no digital signature at its highest (High) macro security level. Conversely, at the highest macro security level (Very High) in Microsoft Excel and Word, you can open a file but all macros will be disabled. At the Medium macro security level (Word and Excel), you can choose to enable or disable macros while opening your file. In Access, there is no option to open a file and disable macros — Access macros are fundamentally different in their nature from Word or Excel macros.
You can verify the authenticity of a digital signature by inspecting its certificate through your Web browser. For a detailed procedure, see the section "Check a digital certificate" in the Quick Reference Card at the end of this course.