but I decided to download it instead

Yes, it's a good solution.

And about this question:

The problem with this is that the PDF shows hieroglyphics, the whole screen broken, instead of the report

You can return a Base64 encoded string like:

[HttpPost("ExportRotativa")]
public string ExportRotativa()
{
    DbImpendingMedicareReportViewModel model = await _IfunctionsUtil.ImpendingMedicareElibigibilityReport();
    model.uIElementModel = SessionManagerUtil.GetUIElement(HttpContext.Session);

    var pdfResult = new ViewAsPdf("NewPdf", model);
    var pdfBytes = await pdfResult.BuildFile(ControllerContext);
    string base64EncodedPDF = System.Convert.ToBase64String(pdfByteArray);
    return base64EncodedPDF;
}

And

$("#hack").on("click", function () {
    win = window.open("", "_blank");
    win.document.write(
        "<iframe width='100%' height='100%' src='data:application/pdf;base64,
            " + encodeURI(htmlData) + "'></iframe> "
    )
 });

Refer to the apply in this link: https://learn.microsoft.com/en-us/answers/questions/976115/how-to-get-a-pdf-file-and-open-in-new-tab-in-asp-n