Generate PDF of only one sheet of my spreadsheet

This question has been addressed more or less here.

In short, you can modify export parameters in Url queryString to get a specific sheet. And then convert the blob to a pdf.

Here is the code for the same, base code was obtained from here

function emailSpreadsheetAsPDF() {

  // Send the PDF of the spreadsheet to this email address
  var email = "Your Email Id"; 

  // Get the currently active spreadsheet URL (link)
  // Or use SpreadsheetApp.openByUrl("<<SPREADSHEET URL>>");
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var sheet = ss.getSheetByName("<Your Sheet name>")

  // Subject of email message
  var subject = "PDF generated from sheet " + sheet.getName(); 

  // Email Body can  be HTML too with your logo image - see ctrlq.org/html-mail
  var body = "Install the <a href="http://www.labnol.org/email-sheet">Email Spreadsheet add-on</a> for one-click conversion.";

  // Base URL
  var url = "https://docs.google.com/spreadsheets/d/SS_ID/export?".replace("SS_ID", ss.getId());

  /* Specify PDF export parameters
  From: https://code.google.com/p/google-apps-script-issues/issues/detail?id=3579
  */

  var url_ext="exportFormat=pdf&format=pdf"        // export as pdf / csv / xls / xlsx
  + '&size=letter'                       // paper size legal / letter / A4
  + '&portrait=false'                    // orientation, false for landscape
  + '&fitw=true&source=labnol'           // fit to page width, false for actual size
  + '&sheetnames=false&printtitle=false' // hide optional headers and footers
  + '&pagenumbers=false&gridlines=false' // hide page numbers and gridlines
  + '&fzr=false'                         // do not repeat row headers (frozen rows) on each page
  + '&gid=';                             // the sheet's Id

  var token = ScriptApp.getOAuthToken();


  //make an empty array to hold your fetched blobs  
  var blobs;


    // Convert your specific sheet to blob
    var response = UrlFetchApp.fetch(url + url_ext + sheet.getSheetId(), {
      headers: {
        'Authorization': 'Bearer ' +  token
      }
    });

    //convert the response to a blob and store in our array
    blobs = response.getBlob().setName(sheet.getName() + '.pdf');


  // Define the scope
  Logger.log("Storage Space used: " + DriveApp.getStorageUsed());

  // If allowed to send emails, send the email with the PDF attachment
  if (MailApp.getRemainingDailyQuota() > 0) 
    GmailApp.sendEmail(email, subject, body, {
      htmlBody: body,
      attachments:[blobs]     
    });  
}

Edit: To combine specific sheets and convert it into and pdf.

Firstly you will have to obtain the sheet object of all the sheets you want to merge into the pdf

var sheet2 = ss.getSheetByName("<Your Sheet name>")
var sheet3 = ss.getSheetByName("<Your Sheet name>")

Then pass the sheetId of each of the sheet to the URL query delimited by %EE%B8%80, Like so

   url += url_ext + sheet.getSheetId()+ "%EE%B8%80"+sheet2.getSheetId()+ "%EE%B8%80"+sheet3.getSheetId()
   var response = UrlFetchApp.fetch(url, {
      headers: {
        'Authorization': 'Bearer ' +  token
      }
    });