I believe your goal is as follows.

• You want to create a PDF data with only the tabs of when the cell "A1" is 1.

In this case, how about the following modification?

Modified script:

function sendReport() {
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var ranges = ss.createTextFinder("1").matchEntireCell(true).findAll();
  var showSheets = ranges.reduce((ar, e) => {
    if (e.getA1Notation() == "A1") ar.push(e.getSheet().getSheetName());
    return ar;
  }, []);
  var sheets = ss.getSheets();
  sheets.forEach(s => {
    if (!showSheets.includes(s.getSheetName())) s.hideSheet();
  });
  SpreadsheetApp.flush();
  var pdf = ss.getBlob().setName("sales_report_2021");

  var message = {
    to: "contact@sexample.com",
    subject: "Sales Reports",
    body: "Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s",
    name: "Sales",
    attachments: [pdf]
  }
  MailApp.sendEmail(message);

  sheets.forEach(s => s.showSheet());
}

• In this modification, the cells which have 1 are retrieved from all sheets using TextFinder. And, the sheets that the cell "A1" has 1 are retrieved. And, the sheets that the cell "A1" has no value of 1 are hidden. And then, the Spreadsheet is converted to a PDF data, and used with MailApp.sendEmail. At last, all sheets are shown. By this flow, your goal is achieved.

References:

• createTextFinder(findText) of Class Spreadsheet
• hideSheet()
• showSheet()