How to get the number of pages of a .PDF uploaded by user?

As has been stated in the other answers, something like pdf.js is be what you are looking for. I've taken a look at the API and it does include a numPages() function to return the total number of pages. It also seems to count pages for me when viewing the demo page from Mozilla.

It depends if you are able to use modern browsers and experimental technology for your solution. pdf.js is very impressive, but it is still experimental according to the github page .

If you are able to count the pages on the server after uploading, then you should look at pdftools or similar.

Something like pdftools --countpages is what you are looking for

In case you use pdf.js you may reference an example on github ('.../examples/node/getinfo.js') with following code that prints number of pages in a pdf file.

const pdfjsLib = require('pdfjs-dist');
...
pdfjsLib.getDocument(pdfPath).then(function (doc) {
    var numPages = doc.numPages;
    console.log('# Document Loaded');
    console.log('Number of Pages: ' + numPages);
})

and a pure javascript solution:

var input = document.getElementById("files");
var reader = new FileReader();
reader.readAsBinaryString(input.files[0]);
reader.onloadend = function(){
    var count = reader.result.match(/\/Type[\s]*\/Page[^s]/g).length;
    console.log('Number of Pages:',count );
}