Unfortunately the search feature does not allow this functionality and it is only designed to search the currently opened publication. However you can search across all your publications by utilising the Library landing page feature. The Library landing page includes a search function that searches within the text of all your publications.
This search function also works from within the Flipbook Viewer app on iOS and Android.
Usage
The library Landing page is created automatically beside your library.json file and is accessible via the Library feature within Flipbooks or directly by navigating to the library.html created next to the library.json file.
Alternatively
We have also prepared the content in such a way that it would be easy for any webmaster to initiate this functionality from your own webpage. For every PDF page that is converted, there is a html page created that contains the text of the PDF page. This will allow search engines to scan the pages.