Google apps script getAs('application/pdf') layout

Try this solution based on https://ctrlq.org/code/19869-email-google-spreadsheets-pdf

Using export url parameters you can set needed options for result pdf. Also you can set specific id of sheet to export, so you don't need to make duplicate of your whole spreadsheet anymore.

function CreaPDF() {
  //The function prints an invoice to PDF. First it copies spreadsheet to a new document.
  //Deletes all sheet except the one to print. Saves it to PDF. 
  //It overwrites any existing doc with same name.

  var sourceSpreadsheet = SpreadsheetApp.getActive();          
  var sheetName = "Factura";
  var folderID = getParentFolder(); // Folder id to save in a folder.
  var sourceSheet = sourceSpreadsheet.getSheetByName(sheetName);
  var folder = DriveApp.getFolderById(folderID); 
  var numf = sourceSpreadsheet.getRangeByName("NumeroFactura").getValue();
  var anof = numf.split("/",2);   // Seeks number and year -> filename 

  var pdfName = anof[1] +"_Factura_" + anof[0]+ "_Dra_Salazar"; // Nombre del documento;

  SpreadsheetApp.getActiveSpreadsheet().toast('Creando PDF');

  // export url
  var url = 'https://docs.google.com/spreadsheets/d/'+sourceSpreadsheet.getId()+'/export?exportFormat=pdf&format=pdf' // export as pdf / csv / xls / xlsx
  + '&size=A4'                           // paper size legal / letter / A4
  + '&portrait=true'                     // orientation, false for landscape
  + '&fitw=false'                        // 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='+sourceSheet.getSheetId();    // the sheet's Id

  var token = ScriptApp.getOAuthToken();

  // request export url
  var response = UrlFetchApp.fetch(url, {
    headers: {
      'Authorization': 'Bearer ' +  token
    }
  });

  var theBlob = response.getBlob().setName(pdfName+'.pdf');

  // delete pdf if already exists
  var files = folder.getFilesByName(pdfName);
  while (files.hasNext())
  {
    files.next().setTrashed(true);
  }

  // create pdf
  var newFile = folder.createFile(theBlob);

  return true;
}

Motivated by Kos' soultion.

I tracked request that created by google spreadsheet Download as GUI with chrome developer Tools. When you click next in Print settings, chrome sents post request and gets pdf file.

Let's cut to the chase. Below is my code.

var sourceSpreadsheet = SpreadsheetApp.getActive();
var url = 'https://docs.google.com/spreadsheets/d/' + sourceSpreadsheet.getId() + '/pdf?id=' + sourceSpreadsheet.getId()
var formData = {
    'a': 'false',
    'pc': '[null,null,null, ... SOME VERY LONG LIST OF NULLS AND NUMBERS ... 166]],[[0,5]]]],0]',
    'gf': '[]'
};
var token = ScriptApp.getOAuthToken(); // doesn't needed when sheet is public
var options = {
        'method': 'post',
        'payload': formData,
        'muteHttpExceptions': true,
        headers: {
            'Authorization': 'Bearer ' + token
        }
    };
var response = UrlFetchApp.fetch(url, options)

var theBlob = response.getBlob().setName('YourPDFName.pdf');

You can fill formData['pc'] by following:
1. Record requests at Network tab.
2. Click pdf?id=blahblah pic for step 2
3. Scroll down to find Form Data pic for step 3
4. Copy form data and fill your code.

Form data looks like:
[null,null,null,null,null,null,null,null,null,0,[["$Sheet_id"]],10000000,null,null,null,null,null,null,null,null,null,null,null,null,null,null,$day_after_30-12-1899,null,null,[$note,null,1,$note,0,0,0,0,$fixed_row,1,1,1,$headers,$footers,1,1],["A4",1,2,1,[0.75,0.75,0.7,0.7]],null,0,[["$Sheet_id",[[26,52]....],[[0,5]]]],0]
There are many more parameters but I can't find them all. Also parameters can be wrong as I found their value by checking changed value.

When you change $day_after_30-12-1899, date printed in pdf changes.

You can customize headers/footers. They are UTF16 encoded and has several control characters.

| Name in `Download as` GUI | character | 
|---------------------------|-----------|
| page number A             | \uee12    |
| page number B             | \uee15    |
| page number C             | \uee16    |
| Workbook title            | \uee10    |
| sheet name                | \uee11    |


|   meaning in date & time  | character | 
|---------------------------|-----------|
| date formatstr start      | \uee13    |
| year                      | yyyy      |
| month                     | M         |
| day                       | d         |
| date formatstr end        | \uee14    |
|                           |           |
| time formatstr start      | \uee17    |
| am or pm                  | am/pm     |
| hour in 12hour clock      | h         |
| minute                    | mm        |

you can use time and date like this YourText\uee13yy. M. d\uee14Yourtext

I'd recommend create formData['pc'] with GUI and modify $day_after_30-12-1899.

*Any corrections are welcome. While I'm not a native English speaker, there are many awkward/incorrect expressions.