As for the technical reason why PB cannot create a full text index of Google docs, I will have to consult with our developers then get back to you.
You can try the following (see below for setup instructions) so that you can search your Google docs from within PB .
1. Login to docs.google.com using your default web browser.
2. In PB press the F4 key to bring up the "Search Web" window.
3. Select the "Edit" button.
4. Select the "Add new" button. You will see a blank name textbox and a blank URL textbox.
5. for Name, put in whatever you desire (eg GoogleDocs). For URL, input the following "https://docs.google.com/#search/TARGETSTRING" (excluding quotes). Then hit the "OK" button.
6. To test it out, hit the F4 key in PB to bring up the "Search Web" window then select "GoogleDocs" (or whatever you added in step 5) in the "Search Engine" drop down. Next, input in the string you want to search for into the text field above the "Search Engine" drop down then select the "OK" button. This will open your web browser then pass the search string into the URL at which point Google Docs will return you its results.
Note: For this to work you have to be logged into Google docs in your default web browser.
Hope this helps.