Without seeing the code, it's impossible to be specific. In general, the key is to retain the page number of the preview page when the user clicks on the link to the product page. This can be done in several different ways, but it all depends on how the underlying software works.
I'd probably recommend using a cookie in this situation because it's likely to be the easiest to integrate into some existing JavaScript. The preview page would always check for the presence of the last-page-number-seen cookie to determine which page to display. If none is found, page 1 is displayed by default. If the cookie is found, then the appropriate page number is displayed. Then the cookie is updated to indicate the last page visited by the user.
Rick Trethewey
Rainbo Design
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