The way Scribus creates bookmarks is unfortunately not the most practical: the bookmark window or sidebar will display the text content of the bookmarked frame. So if you bookmark a frame with a long text, it becomes unusable. What we usually do is:
Display the Layers window from Windows | Layers.
Create a layer in the Layers window by clicking on the + at the bottom left.
Browse to a page or frame that needs to be bookmarked and draw a new Text Frame over it.
Inside that frame, write the text you want to display in the bookmark sidebar, generally a small text like a title.
You can set the Opacity of this layer to 0%.
Then, when exporting to PDF, enable the Include Bookmarks option of the General tab and choose at least the 1.4 version of PDF to keep transparencies.
If you want the tab to be displayed when the reader opens your PDF, just enable Display Bookmarks Tab in the Viewer tab of the PDF export window.