Hello Daniel,
It’s a timing issue, so maybe try to put your page code like this:
import wixWindow from ‘wix-window’ ;
$w.onReady(function () {
if (wixWindow.rendering.env === “browser”) {
let file = { filepath: " https://yourfiledirecturl.pdf “, filename: " filename.pdf “};
$w(”#html1”).postMessage(file)
}
});
And in the html component add “filename” to event.data, like this:
“addEventListener(‘message’, event => {if (event.data.filename){….”
Let me know if it worked out 
Cristina