Fix: reduce sdk version to fit pdf reader

build.gradle

@@ -7,7 +7,7 @@ android {
 
 
     defaultConfig {
-        minSdkVersion 21
+        minSdkVersion 15
         targetSdkVersion 28
         versionCode 1
         versionName "1.0"
@@ -28,7 +28,7 @@ android {
 dependencies {
     implementation fileTree(dir: 'libs', include: ['*.jar'])
 
-    implementation 'com.android.support:appcompat-v7:28.0.0'
+    implementation 'com.android.support:appcompat-v7:27.1.1'
     testImplementation 'junit:junit:4.12'
     androidTestImplementation 'com.android.support.test:runner:1.0.2'
     androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'