import sys
from PyQt5.QtCore import QProcess
from PyQt5.QtWidgets import QApplication, QMainWindow, QLabel


class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()

        self.container = QLabel("PDF Area")
        self.process = QProcess()

        self.setCentralWidget(self.container)

    def run(self, command):
        '''
            ....

            Code to attach command output ie a GUI window inside self.container

            ....

        '''
        
        self.process.start(command)


app = QApplication(sys.argv)

window = MainWindow()
window.show()
window.run('evince -s mypdf.pdf')
app.exec()