diff --git a/CHANGELOG.md b/CHANGELOG.md
index eb0e8d2..db0d0fe 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+- Internal changes in `PdfDocument` and `PdfViewerPreferences` classes.
 - Added `SymfonyInsight` quality check tool, configuration and badge.
 - Added descriptions to `PdfBlendMode` enumeration.
 - Minor changes in tutorials.
diff --git a/src/PdfDocument.php b/src/PdfDocument.php
index 1b67893..d909dc6 100644
--- a/src/PdfDocument.php
+++ b/src/PdfDocument.php
@@ -563,9 +563,9 @@ public function cell(
         $border ??= PdfBorder::none();
         if ($fill || $border->isAll()) {
             if ($fill) {
-                $op = $border->isAll() ? 'B' : 'f';
+                $style = $border->isAll() ? PdfRectangleStyle::BOTH : PdfRectangleStyle::FILL;
             } else {
-                $op = 'S';
+                $style = PdfRectangleStyle::BORDER;
             }
             $output .= \sprintf(
                 '%.2F %.2F %.2F %.2F re %s ',
@@ -573,7 +573,7 @@ public function cell(
                 ($this->height - $this->y) * $scaleFactor,
                 $width * $scaleFactor,
                 -$height * $scaleFactor,
-                $op
+                $style->value
             );
         }
         if ('' === $output && $border->isAny()) {
diff --git a/src/PdfViewerPreferences.php b/src/PdfViewerPreferences.php
index ed8d42b..822a35d 100644
--- a/src/PdfViewerPreferences.php
+++ b/src/PdfViewerPreferences.php
@@ -366,11 +366,11 @@ private function getOutputBool(bool $value, string $key): string
     }
 
     /**
-     * @phpstan-template T of \BackedEnum&PdfEnumDefaultInterface
+     * @phpstan-template T of PdfEnumDefaultInterface&\BackedEnum
      *
-     * @phpstan-param (T&PdfEnumDefaultInterface<T>) $enum
+     * @phpstan-param PdfEnumDefaultInterface<T>&\BackedEnum $enum
      */
-    private function getOutputEnum(\BackedEnum&PdfEnumDefaultInterface $enum, string $key): string
+    private function getOutputEnum(PdfEnumDefaultInterface $enum, string $key): string
     {
         return $enum->isDefault() ? '' : \sprintf('/%s %s', $key, $enum->value);
     }