Select the "Enable Article Preview" option to let users preview any retail folio that includes articles marked as "Free" (use the Folio Producer Editor to mark folios as Free). When users tap the cover image of a retail folio that has free articles in the library, a Preview button appears in the preview pane below the Buy and Subscribe (if available) buttons. When users tap Preview, all articles marked as Free are downloaded, along with preview images of protected article. When users go into browse mode or swipe to view protected articles, a paywall pane covers most of each protected article.
At this time, the Free Article Preview feature is iPad only, works only in retail folios, and does not support folios that include Sections.
In terms of publisher download counts, free articles are charged against download counts based on the number of free articles downloaded and the total number of articles in the retail folio. If a retail folio includes 20 total articles with 5 free preview articles, 1 download is counted when 4 users preview the folio.
The Paywall Threshold setting under Device Viewer allows your customers to download and view the specified number of articles marked as "Metered" when previewing articles in a retail folio. Use the Folio Producer Editor to mark articles as Metered. Once a user reaches the limit specified in Paywall Threshold, a paywall message appears when they browse to either Metered or Protected articles. The threshold you specify for the app is applied to each folio. If you set the threshold to 3, users can preview 3 metered articles in each folio, and a paywall appears when they navigate to the fourth metered article.
For more detailed information, see the DPS Developer Center article about Free Article Preview.