code: function PDFDocument_checkHeader() {
        var stream = this.stream;
        stream.reset();
        if (find(stream, '%PDF-', 1024)) {
          // Found the header, trim off any garbage before it.
          stream.moveStart();
          // Reading file format version
          var MAX_VERSION_LENGTH = 12;
          var version = '', ch;
          while ((ch = stream.getByte()) > 0x20) { // SPACE
            if (version.length >= MAX_VERSION_LENGTH) {
              break;
            }
            version += String.fromCharCode(ch);
          }
          if (!this.pdfFormatVersion) {
            // removing "%PDF-"-prefix
            this.pdfFormatVersion = version.substring(5);
          }
          return;
        }
        // May not be a PDF file, continue anyway.
      }
docs: Find the header, remove leading garbage and setup the stream starting from the header.