Передача PyQt5.QtCore.QMetaObject.Connection object в строку

Вы не предоставили примеры файлов, которые вы читаете,
поэтому я не проверил работу приложения.

Вы не правильно понимаете работу сигналов и слотов, почитайте Signals & Slots

import sys
from PyQt5 import QtCore, QtGui, QtWidgets

#from ui import Ui_Main
class Ui_Main(object):
    def setupUi(self, Main):
        Main.setObjectName("Main")
        Main.setWindowModality(QtCore.Qt.ApplicationModal)
        Main.resize(400, 282)
        icon = QtGui.QIcon()
        icon.addPixmap(QtGui.QPixmap("copy.ico"), QtGui.QIcon.Normal, QtGui.QIcon.Off)
        Main.setWindowIcon(icon)
        Main.setAccessibleName("")
        self.verticalLayout_3 = QtWidgets.QVBoxLayout(Main)
        self.verticalLayout_3.setObjectName("verticalLayout_3")
        self.verticalLayout = QtWidgets.QVBoxLayout()
        self.verticalLayout.setObjectName("verticalLayout")
        self.label = QtWidgets.QLabel(Main)
        self.label.setAlignment(QtCore.Qt.AlignCenter)
        self.label.setObjectName("label")
        self.verticalLayout.addWidget(self.label)
        self.verticalLayout_3.addLayout(self.verticalLayout)
        self.verticalLayout_6 = QtWidgets.QVBoxLayout()
        self.verticalLayout_6.setObjectName("verticalLayout_6")
        self.label_3 = QtWidgets.QLabel(Main)
        self.label_3.setObjectName("label_3")
        self.verticalLayout_6.addWidget(self.label_3)
        self.verticalLayout_3.addLayout(self.verticalLayout_6)
        self.horizontalLayout = QtWidgets.QHBoxLayout()
        self.horizontalLayout.setObjectName("horizontalLayout")
        self.pushButton = QtWidgets.QPushButton(Main)
        self.pushButton.setObjectName("pushButton")
        self.horizontalLayout.addWidget(self.pushButton)
        self.lineEdit = QtWidgets.QLineEdit(Main)
        self.lineEdit.setClearButtonEnabled(False)
        self.lineEdit.setObjectName("lineEdit")
        self.horizontalLayout.addWidget(self.lineEdit)
        self.verticalLayout_3.addLayout(self.horizontalLayout)
        self.verticalLayout_4 = QtWidgets.QVBoxLayout()
        self.verticalLayout_4.setObjectName("verticalLayout_4")
        self.label_5 = QtWidgets.QLabel(Main)
        self.label_5.setObjectName("label_5")
        self.verticalLayout_4.addWidget(self.label_5)
        self.verticalLayout_3.addLayout(self.verticalLayout_4)
        self.horizontalLayout_2 = QtWidgets.QHBoxLayout()
        self.horizontalLayout_2.setObjectName("horizontalLayout_2")
        self.pushButton_2 = QtWidgets.QPushButton(Main)
        self.pushButton_2.setObjectName("pushButton_2")
        self.horizontalLayout_2.addWidget(self.pushButton_2)
        self.lineEdit_2 = QtWidgets.QLineEdit(Main)
        self.lineEdit_2.setClearButtonEnabled(False)
        self.lineEdit_2.setObjectName("lineEdit_2")
        self.horizontalLayout_2.addWidget(self.lineEdit_2)
        self.verticalLayout_3.addLayout(self.horizontalLayout_2)
        self.verticalLayout_5 = QtWidgets.QVBoxLayout()
        self.verticalLayout_5.setObjectName("verticalLayout_5")
        self.label_4 = QtWidgets.QLabel(Main)
        self.label_4.setObjectName("label_4")
        self.verticalLayout_5.addWidget(self.label_4)
        self.verticalLayout_3.addLayout(self.verticalLayout_5)
        self.horizontalLayout_3 = QtWidgets.QHBoxLayout()
        self.horizontalLayout_3.setObjectName("horizontalLayout_3")
        self.pushButton_4 = QtWidgets.QPushButton(Main)
        self.pushButton_4.setObjectName("pushButton_4")
        self.horizontalLayout_3.addWidget(self.pushButton_4)
        self.lineEdit_3 = QtWidgets.QLineEdit(Main)
        self.lineEdit_3.setClearButtonEnabled(False)
        self.lineEdit_3.setObjectName("lineEdit_3")
        self.horizontalLayout_3.addWidget(self.lineEdit_3)
        self.verticalLayout_3.addLayout(self.horizontalLayout_3)
        self.verticalLayout_2 = QtWidgets.QVBoxLayout()
        self.verticalLayout_2.setObjectName("verticalLayout_2")
        self.pushButton_3 = QtWidgets.QPushButton(Main)
        self.pushButton_3.setObjectName("pushButton_3")
        self.verticalLayout_2.addWidget(self.pushButton_3)
        self.verticalLayout_3.addLayout(self.verticalLayout_2)

        self.retranslateUi(Main)
        QtCore.QMetaObject.connectSlotsByName(Main)

    def retranslateUi(self, Main):
        _translate = QtCore.QCoreApplication.translate
        Main.setWindowTitle(_translate("Main", "DXF-CP"))
        self.label.setText(_translate("Main", "Generating a dxf file by adding data\n"
" to the destination file from the donor file"))
        self.label_3.setText(_translate("Main", "Donor file:"))
        self.pushButton.setText(_translate("Main", "Open"))
        self.label_5.setText(_translate("Main", "Destination file:"))
        self.pushButton_2.setText(_translate("Main", "Open"))
        self.label_4.setText(_translate("Main", "Output directory:"))
        self.pushButton_4.setText(_translate("Main", "Open"))
        self.pushButton_3.setText(_translate("Main", "Generate"))
        

class MainWindow(QtWidgets.QWidget, Ui_Main):
    def __init__(self):
        super().__init__()
        
        self.setupUi(self)
        
        self.file_donor = None
        self.file_getter = None
        self.directory = None
        
        self.pushButton.clicked.connect(self.get_file_donor)
        self.pushButton_2.clicked.connect(self.get_file_getter)
        self.pushButton_4.clicked.connect(self.get_output_directory)
        
        self.pushButton_3.clicked.connect(self.copypaste)

    def get_file_donor(self):
        self.file_donor = QtWidgets.QFileDialog().getOpenFileName()[0]
        self.lineEdit.setText(self.file_donor)

    def get_file_getter(self):
        self.file_getter = QtWidgets.QFileDialog().getOpenFileName()[0]
        self.lineEdit_2.setText(self.file_getter)

    def get_output_directory(self):
        self.directory = QtWidgets.QFileDialog().getExistingDirectory()
        self.lineEdit_3.setText(self.directory)

    def copypaste(self):
        if not self.file_donor or not self.file_getter or not self.directory:
            return
        
        string = '  0\n'
        file_1 = open(self.file_donor, encoding='utf-8')
        data_1 = file_1.readlines()
        data_1 = data_1[data_1.index("POLYLINE\n"):]
        file_2 = open(self.file_getter, encoding='utf-8')
        data_2 = file_2.readlines()
        file_3 = open(self.directory + '/file_output.dxf', 'a', encoding='utf-8')

        for el in data_2[:data_2.index("EOF\n")]:
            file_3.write(el)
            
        for el in data_1[:data_1.index("ENDSEC\n")]:
            file_3.write(el)
            
        file_3.write('ENDSEC\n')
        file_3.write(string)
        file_3.write("EOF\n")

        file_1.close()
        file_2.close()
        file_3.close()


if __name__ == "__main__":
    import sys
    app = QtWidgets.QApplication(sys.argv)
    w = MainWindow()
    w.show()
    sys.exit(app.exec_())