Right click and show package contents in Safari. Then open Info.plist in TextEdit. Find the first document type (the second overall) and paste this right before it; you're essentially adding a document type key:
<dict>Save, run Safari, quit Safari, and then enjoy drag and drop PDFs in Safari.
<key>CFBundleTypeExtensions</key>
<array>
<string>pdf</string>
<string>PDF</string>
</array>
<key>CFBundleTypeMIMETypes</key>
<array>
<string>application/pdf</string>
</array>
<key>CFBundleTypeName</key>
<string>NSPDFPboardType</string>
<key>CFBundleTypeOSTypes</key>
<array>
<string>PDF </string>
</array>
<key>CFBundleTypeRole</key>
<string>Viewer</string>
<key>NSDocumentClass</key>
<string>BrowserDocument</string>
</dict>
[Editor's note: I have not tested this one...]

