<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <LinearLayout
        android:id="@+id/wrapper"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:gravity="center_horizontal"
        android:orientation="horizontal">

        <Button
            android:id="@+id/increase"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Increase" />

        <Button
            android:id="@+id/decrease"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Decrease" />
    </LinearLayout>

    <ScrollView
        android:id="@+id/scroll"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_above="@id/wrapper"
        android:layout_centerInParent="true">

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="vertical">

        </LinearLayout>
    </ScrollView>

    <TextView
        android:id="@+id/tv_text"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentEnd="true"
        android:layout_marginTop="1dp"
        android:layout_marginEnd="0dp"
        android:text="Lorem ipsum dolor sit amet, consectetur adipiscing elit. Duis sem augue,
                      aliquam bibendum fringilla quis, volutpat ut arcu. Sed nulla metus, gravida
                      id pulvinar quis, rhoncus in velit. Pellentesque semper mollis leo,
                      vitae molestie risus. Curabitur nec suscipit tortor. Quisque non purus eu
                      quam pretium mollis sed in turpis. Duis elit magna, ullamcorper vitae elementum
                      in, auctor eget ligula. Maecenas ultricies diam non nisl facilisis porta.
                      Suspendisse diam ante, accumsan sit amet enim nec, bibendum semper arcu.
                      Nunc a imperdiet odio. Morbi id est finibus ex mollis interdum vulputate non
                      eros. Interdum et malesuada fames ac ante ipsum primis in faucibus. Vestibulum 
                      sit amet dictum ante, vitae condimentum augue. Proin ultricies enim nisl,
                      eu pharetra arcu venenatis sit amet. Pellentesque sodales, justo eu iaculis
                      rhoncus, magna mi ullamcorper enim, a mattis neque sapien eu nisi. Duis a 
                      turpis euismod nibh mattis egestas sed vel sem. Maecenas non tempor tellus,
                      id facilisis erat. Nullam id commodo nisi. Ut sed arcu lectus. Mauris lacus 
                      libero, pharetra et neque vitae, tincidunt dapibus magna. Sed non scelerisque 
                      leo, non pharetra mi. In sollicitudin metus ut lacus vestibulum efficitur.
                      Sed cursus pellentesque ante at vehicula. Nunc eros metus, mattis at aliquet at,
                      euismod et libero.!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</RelativeLayout>

import android.os.Bundle;
import android.util.TypedValue;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {

    Button increase, decrease;
    private float ourFontsize = 14f;
    TextView text;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        increase = findViewById(R.id.increase);
        decrease = findViewById(R.id.decrease);
        text = findViewById(R.id.tv_text);
        increase.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // increasing the size by 4 unit
                ourFontsize += 4f;
                // assigning new textsize to our text
                text.setTextSize(TypedValue.COMPLEX_UNIT_SP, ourFontsize);
            }
        });
        decrease.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // decreasing the size by 4 unit
                ourFontsize -= 4f;
                // assigning new textsize to our text
                text.setTextSize(TypedValue.COMPLEX_UNIT_SP, ourFontsize);
            }
        });
    }
}