Cool reader is unable to open some file types from other applications (e.g. Dropbox or a file explorer). I have tested a change which seems to solve this for mobi and pdb files; simply add the following 2 lines to the relevant <intent-filter> in AndroidManifest.xml:
<data android:mimeType="application/x-mobipocket-ebook" />
<data android:mimeType="chemical/x-pdb" />