Instead of downloading, an exported PDF can be shown inline in the browser — render the report, export to a PDF blob, and open it in a new tab.

Render, then export to a PDF blob.
var report = new Stimulsoft.Report.StiReport();
report.loadFile("../reports/SimpleList.mrt");
report.renderAsync2();

function saveReportPdf() {
    report.exportDocumentAsync(function (pdfData) {
        var blob = new Blob([new Uint8Array(pdfData)], { type: "application/pdf" });
        window.open(URL.createObjectURL(blob));
    }, Stimulsoft.Report.StiExportFormat.Pdf);
}

How it works. The PDF is produced entirely client-side and shown from a blob URL, so no server round-trip or file download is required.
На скриншоте ниже Вы можете увидеть результат выполнения данного кода:

Showing a Report as PDF