Нужно, чтобы при нажатии self.printAction открывалось окно MyWindow (чтобы в открывшемся окне функционировал принтер, который уже написан)

import sys

from PyQt6.QtCore import Qt
from PyQt6.QtWidgets import QApplication, QLabel, QMainWindow, \\
    QMenu, QToolBar,QTextEdit, QVBoxLayout, QPushButton

from PyQt5 import QtCore, QtWidgets, QtGui, QtPrintSupport

from PyQt5.Qt import *

class Window(QMainWindow):
    def __init__(self, parent=None):
        super().__init__(parent)

        self.setWindowTitle("Python Menus&")
        self.resize(400, 200)

        self.centralWidget = QWidget(objectName='centralWidget')

        self.setCentralWidget(self.centralWidget)

        self.centralWidget.setStyleSheet("color: rgb(255, 90, 58); background-color: #105652; ")
        self.text = QTextEdit('Hello! Write here!')

        layout = QVBoxLayout(self.centralWidget)
        layout.addWidget(self.text)

        self._createActions()
        self._createMenuBar()

    def _createMenuBar(self):
        menuBar = self.menuBar()
        fileMenu = QMenu("&File", self)
        menuBar.addMenu(fileMenu)
        fileMenu.addAction(self.newAction)
        fileMenu.addAction(self.openAction)
        fileMenu.addAction(self.saveAction)
        fileMenu.addAction(self.exitAction)
        editMenu = menuBar.addMenu("&Edit")
        editMenu.addAction(self.copyAction)
        editMenu.addAction(self.pasteAction)
        editMenu.addAction(self.cutAction)
        helpMenu = menuBar.addMenu("&Help")
        helpMenu.addAction(self.helpContentAction)
        helpMenu.addAction(self.aboutAction)
        printMenu = menuBar.addMenu("&Print")
        printMenu.addAction(self.printAction)

    def _createActions(self):
        self.newAction = QAction(self)
        self.newAction.setText("New")
        self.openAction = QAction("Open", self)
        self.saveAction = QAction("Save", self)
        self.exitAction = QAction("Exit", self)
        self.copyAction = QAction("&Copy", self)
        self.pasteAction = QAction("&Paste", self)
        self.cutAction = QAction("Cut", self)
        self.helpContentAction = QAction("Help", self)
        self.aboutAction = QAction("About", self)
        self.printAction = QAction("Print", self)
        self.printAction.triggered.connect(self.show_window_1)

    def show_window_1(self):
        self.w1 = MyWindow()
        self.w1.show()

class MyWindow(QtWidgets.QDialog):
    def __init__(self, parent=None):
        QtWidgets.QWidget.__init__(self, parent, flags=QtCore.Qt.Window |
        QtCore.Qt.MSWindowsFixedSizeDialogHint)
        self.setWindowTitle("Печать изображений")

        self.text = QTextEdit('Type your text here')
        self.button = QPushButton('Print', self)
        self.button.clicked.connect(self.func)

        vbox = QtWidgets.QVBoxLayout()
        vbox.addWidget(self.text)
        vbox.addWidget(self.button)

        self.setLayout(vbox)
        self.resize(200, 100)

    def func(self):
        self.setWindowTitle("Печать изображений")
        self.printer = QtPrintSupport.QPrinter()
        self.printer.setPageOrientation(QtGui.QPageLayout.Landscape)
        self.file = None
        self.preview()

    def showPageOptions(self):
        pd = QtPrintSupport.QPageSetupDialog(self.printer, parent=self)
        pd.exec()

    def preview(self):
        pp = QtPrintSupport.QPrintPreviewDialog(self.printer, parent=self)
        pp.paintRequested.connect(self._printText)
        pp.exec()

    def print(self):
        pd = QtPrintSupport.QPrintDialog(self.printer, parent=self)
        pd.setOptions(
            QtPrintSupport.QAbstractPrintDialog.PrintToFile | QtPrintSupport.QAbstractPrintDialog.PrintSelection)
        if pd.exec() == QtWidgets.QDialog.Accepted:
            self._printImage(self.printer)

        self.show()

    def _printText(self, printer):
        painter = QtGui.QPainter()
        painter.begin(printer)
        color = QtGui.QColor(QtCore.Qt.black)
        painter.setPen(QtGui.QPen(color))
        painter.setBrush(QtGui.QBrush(color))
        font = QtGui.QFont("Verdana", pointSize=12)
        painter.setFont(font)
        painter.drawText(printer.width(), printer.height(), QtCore.Qt.TextDontClip, self.text.toPlainText())
        painter.end()

if __name__ == "__main__":
    app = QApplication(sys.argv)
    win = Window()
    win.show()
    sys.exit(app.exec())

Не уверен, что правильно понял вас, но попробуйте так:

import sys
from PyQt5 import QtCore, QtWidgets, QtGui, QtPrintSupport
from PyQt5.Qt import *


class MyWindow(QtWidgets.QDialog):
    def __init__(self, parent=None):
        super(MyWindow, self).__init__(parent,                          # +
            flags=Qt.Window | Qt.MSWindowsFixedSizeDialogHint)
            
        self.parent = parent                                            # +++
        
        self.setWindowTitle("Печать изображений")
        
        self.func()                                                     # +++
        
    def func(self):
        self.setWindowTitle("Печать изображений")
        self.printer = QtPrintSupport.QPrinter()
        self.printer.setPageOrientation(QtGui.QPageLayout.Landscape)
        
        self.file = None
        vbox = QtWidgets.QVBoxLayout()
        btnOpen = QtWidgets.QPushButton("&Открыть файл...")
        btnOpen.clicked.connect(self.openFile)                            # ???
        vbox.addWidget(btnOpen)
        
        btnPageOptions = QtWidgets.QPushButton("Настройка &страницы...")
        btnPageOptions.clicked.connect(self.showPageOptions)
        vbox.addWidget(btnPageOptions)
        btnPreview = QtWidgets.QPushButton("П&росмотр...")
        btnPreview.clicked.connect(self.preview)
        vbox.addWidget(btnPreview)
        btnPrint = QtWidgets.QPushButton("&Печать...")
        btnPrint.clicked.connect(self._print)
        vbox.addWidget(btnPrint)
        self.setLayout(vbox)
        self.resize(200, 100)
        
    def openFile(self):                                                  # ???
        pass

    def showPageOptions(self):
        pd = QtPrintSupport.QPageSetupDialog(self.printer, parent=self)
        pd.exec()

    def preview(self):
        pp = QtPrintSupport.QPrintPreviewDialog(self.printer, parent=self)
        pp.paintRequested.connect(self._printText)
        pp.exec()

    def _print(self):
        pd = QtPrintSupport.QPrintDialog(self.printer, parent=self)
        pd.setOptions(
            QtPrintSupport.QAbstractPrintDialog.PrintToFile | QtPrintSupport.QAbstractPrintDialog.PrintSelection)
        if pd.exec() == QtWidgets.QDialog.Accepted:
            self._printImage(self.printer)

        self.show()

    def _printText(self, printer):
        painter = QtGui.QPainter()
        painter.begin(printer)
        color = QtGui.QColor(QtCore.Qt.black)
        painter.setPen(QtGui.QPen(color))
        painter.setBrush(QtGui.QBrush(color))
        font = QtGui.QFont("Verdana", pointSize=42)
        painter.setFont(font)
        #pixmap = QtGui.QPixmap(self.file)
        
        painter.drawText(
#            printer.width(), 
#            printer.height(), 
#            QtCore.Qt.TextDontClip, 
#            self.text.text()
            42,                                                       # +++
            42,                                                       # +++
            self.parent.text.toPlainText()                            # +++
        )
        painter.end()
        

class Window(QMainWindow):
    def __init__(self, parent=None):
        super().__init__(parent)

        self.setWindowTitle("Python Menus&")
        self.resize(400, 200)

        self.centralWidget = QWidget(objectName='centralWidget')
        self.setCentralWidget(self.centralWidget)
        self.centralWidget.setStyleSheet("color: rgb(255, 90, 58); background-color: #105652; ")
        self.text = QTextEdit('Hello! Write here!')

        layout = QVBoxLayout(self.centralWidget)
        layout.addWidget(self.text)

        self._createActions()
        self._createMenuBar()

    def _createMenuBar(self):
        menuBar = self.menuBar()
        fileMenu = QMenu("&File", self)
        menuBar.addMenu(fileMenu)
        fileMenu.addAction(self.newAction)
        fileMenu.addAction(self.openAction)
        fileMenu.addAction(self.saveAction)
        fileMenu.addAction(self.exitAction)
        editMenu = menuBar.addMenu("&Edit")
        editMenu.addAction(self.copyAction)
        editMenu.addAction(self.pasteAction)
        editMenu.addAction(self.cutAction)
        helpMenu = menuBar.addMenu("&Help")
        helpMenu.addAction(self.helpContentAction)
        helpMenu.addAction(self.aboutAction)
        printMenu = menuBar.addMenu("&Print")
        printMenu.addAction(self.printAction)

    def _createActions(self):
        self.newAction = QAction(self)
        self.newAction.setText("New")
        self.openAction = QAction("Open", self)
        self.saveAction = QAction("Save", self)
        self.exitAction = QAction("Exit", self)
        self.copyAction = QAction("&Copy", self)
        self.pasteAction = QAction("&Paste", self)
        self.cutAction = QAction("Cut", self)
        self.helpContentAction = QAction("Help", self)
        self.aboutAction = QAction("About", self)
        self.printAction = QAction("Print", self)
        
        self.printAction.triggered.connect(self.show_window_1)

    def show_window_1(self):
        self.w1 = MyWindow(self)                                  # +++  (self)
        self.w1.show()


if __name__ == "__main__":
    app = QApplication(sys.argv)
    win = Window()
    win.show()
    sys.exit(app.exec())