You could edit the _canonical_uri url with a button with the appearance of a link that open it, e.g :
In a tiddler with the tag $:/tags/Macro/View
\define open-pdf(tiddler,page)
<$tiddler tiddler=<<__tiddler__>>>
<$button class="tc-tiddlylink tc-tiddlylink-resolves" tag="a" set="!!_canonical_uri" setTo={{{ [{!!_canonical_uri}split[#page=]first[]] [[#page=]] [<__page__>] +[join[]] }}} actions="<$action-navigate/>">
Open "{{!!title}}" to page $page$
</$button>
</$tiddler>
\end
<<open-pdf tiddler:"pdf" page:"4">>
If you want your pdfs to reset at startup then you could add a startup action. In a tiddler with the tag $:/tags/StartupAction :
<$list filter="[type[application/pdf]search:_canonical_uri[#page=]]">
<$action-setfield _canonical_uri={{{ [{!!_canonical_uri}split[#page=]first[]] }}}/>
</$list>
Make a backup of your wiki before trying this !