diff --git a/lib/AppConfig.php b/lib/AppConfig.php
index 3e0ee1ddb0..5e5f1e1cfb 100644
--- a/lib/AppConfig.php
+++ b/lib/AppConfig.php
@@ -30,9 +30,14 @@ class AppConfig {
 	// Default: 'no', set to 'yes' to enable
 	public const USE_SECURE_VIEW_ADDITIONAL_MIMES = 'use_secure_view_additional_mimes';
 
+	public const PREVIEW_CONVERSION_TIMEOUT = 'preview_conversion_timeout';
+	public const PREVIEW_CONVERSION_MAX_FILESIZE = 'preview_conversion_max_filesize';
+
 	private array $defaults = [
 		'wopi_url' => '',
 		'timeout' => 15,
+		'preview_conversion_timeout' => 5,
+		'preview_conversion_max_filesize' => 104857600, // 100 MB
 		'watermark_text' => '{userId}',
 		'watermark_allGroupsList' => [],
 		'watermark_allTagsList' => [],
diff --git a/lib/Preview/Office.php b/lib/Preview/Office.php
index 8b8caa8390..7b90da76ae 100644
--- a/lib/Preview/Office.php
+++ b/lib/Preview/Office.php
@@ -33,12 +33,22 @@ public function isAvailable(FileInfo $file): bool {
 	}
 
 	public function getThumbnail(File $file, int $maxX, int $maxY): ?IImage {
-		if ($file->getSize() === 0) {
+		$fileSize = $file->getSize();
+		if ($fileSize === 0) {
+			return null;
+		}
+
+		$maxFileSize = $this->appConfig->getPreviewConversionMaxFileSize();
+		if ($fileSize > $maxFileSize) {
+			$this->logger->debug('Skipping preview conversion: file size {size} exceeds limit {limit}', [
+				'size' => $fileSize,
+				'limit' => $maxFileSize,
+			]);
 			return null;
 		}
 
 		try {
-			$response = $this->remoteService->convertFileTo($file, 'png');
+			$response = $this->remoteService->convertFileTo($file, 'png', $this->appConfig->getPreviewConversionTimeout());
 			$image = new Image();
 			$image->loadFromData($response);
 
diff --git a/lib/Service/RemoteService.php b/lib/Service/RemoteService.php
index ac2df4d102..ffd24d9b07 100644
--- a/lib/Service/RemoteService.php
+++ b/lib/Service/RemoteService.php
@@ -57,14 +57,19 @@ public function fetchTargetThumbnail(File $file, string $target): ?string {
 	/**
 	 * @return resource|string
 	 */
-	public function convertFileTo(File $file, string $format) {
+	public function convertFileTo(File $file, string $format, int $timeout = RemoteOptionsService::REMOTE_TIMEOUT_DEFAULT) {
 		$fileName = $file->getStorage()->getLocalFile($file->getInternalPath());
 		$stream = fopen($fileName, 'rb');
 
 		if ($stream === false) {
 			throw new Exception('Failed to open stream');
 		}
-		return $this->convertTo($file->getName(), $stream, $format);
+
+		try {
+			return $this->convertTo($file->getName(), $stream, $format, [], $timeout);
+		} finally {
+			fclose($stream);
+		}
 	}
 
 	/**
@@ -73,7 +78,7 @@ public function convertFileTo(File $file, string $format) {
 	 */
 	public function convertTo(string $filename, $stream, string $format) {
 		$client = $this->clientService->newClient();
-		$options = RemoteOptionsService::getDefaultOptions();
+		$options = RemoteOptionsService::getDefaultOptions($timeout);
 		// FIXME: can be removed once https://github.com/CollaboraOnline/online/issues/6983 is fixed upstream
 		$options['expect'] = false;
 
diff --git a/tests/lib/AppConfigTest.php b/tests/lib/AppConfigTest.php
index 1f9296427f..c9722f1fff 100644
--- a/tests/lib/AppConfigTest.php
+++ b/tests/lib/AppConfigTest.php
@@ -58,4 +58,40 @@ public function testGetAppValueArrayWithNoneValue() {
 
 		$this->assertSame([], $result);
 	}
+
+	public function testGetPreviewConversionTimeoutReturnsDefault(): void {
+		$this->config->expects($this->once())
+			->method('getAppValue')
+			->with('richdocuments', 'preview_conversion_timeout', 5)
+			->willReturn(5);
+
+		$this->assertSame(5, $this->appConfig->getPreviewConversionTimeout());
+	}
+
+	public function testGetPreviewConversionTimeoutReturnsConfiguredValue(): void {
+		$this->config->expects($this->once())
+			->method('getAppValue')
+			->with('richdocuments', 'preview_conversion_timeout', 5)
+			->willReturn(30);
+
+		$this->assertSame(30, $this->appConfig->getPreviewConversionTimeout());
+	}
+
+	public function testGetPreviewConversionMaxFileSizeReturnsDefault(): void {
+		$this->config->expects($this->once())
+			->method('getAppValue')
+			->with('richdocuments', 'preview_conversion_max_filesize', 104857600)
+			->willReturn(104857600);
+
+		$this->assertSame(104857600, $this->appConfig->getPreviewConversionMaxFileSize());
+	}
+
+	public function testGetPreviewConversionMaxFileSizeReturnsConfiguredValue(): void {
+		$this->config->expects($this->once())
+			->method('getAppValue')
+			->with('richdocuments', 'preview_conversion_max_filesize', 104857600)
+			->willReturn(52428800);
+
+		$this->assertSame(52428800, $this->appConfig->getPreviewConversionMaxFileSize());
+	}
 }
diff --git a/tests/lib/Preview/OfficeTest.php b/tests/lib/Preview/OfficeTest.php
new file mode 100644
index 0000000000..f3bb6d19cc
--- /dev/null
+++ b/tests/lib/Preview/OfficeTest.php
@@ -0,0 +1,103 @@
+<?php
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace Tests\Richdocuments\Preview;
+
+use OCA\Richdocuments\AppConfig;
+use OCA\Richdocuments\Capabilities;
+use OCA\Richdocuments\Preview\Office;
+use OCA\Richdocuments\Service\RemoteService;
+use OCP\Files\File;
+use PHPUnit\Framework\MockObject\MockObject;
+use PHPUnit\Framework\TestCase;
+use Psr\Log\LoggerInterface;
+
+class OfficeTest extends TestCase {
+	private RemoteService&MockObject $remoteService;
+	private LoggerInterface&MockObject $logger;
+	private AppConfig&MockObject $appConfig;
+	private Capabilities&MockObject $capabilities;
+	private Office $provider;
+
+	protected function setUp(): void {
+		parent::setUp();
+
+		$this->remoteService = $this->createMock(RemoteService::class);
+		$this->logger = $this->createMock(LoggerInterface::class);
+		$this->appConfig = $this->createMock(AppConfig::class);
+		$this->capabilities = $this->createMock(Capabilities::class);
+		$this->capabilities->method('getCapabilities')->willReturn(['richdocuments' => []]);
+
+		$this->provider = new class($this->remoteService, $this->logger, $this->appConfig, $this->capabilities) extends Office {
+			#[\Override]
+			public function getMimeType(): string {
+				return '/application\\/test/';
+			}
+		};
+	}
+
+	public function testGetThumbnailSkipsConversionWhenFileIsTooLarge(): void {
+		$file = $this->createMock(File::class);
+		$file->expects($this->once())->method('getSize')->willReturn(101 * 1024 * 1024);
+
+		$this->appConfig->expects($this->once())
+			->method('getPreviewConversionMaxFileSize')
+			->willReturn(100 * 1024 * 1024);
+		$this->remoteService->expects($this->never())->method('convertFileTo');
+
+		$result = $this->provider->getThumbnail($file, 64, 64);
+
+		$this->assertNull($result);
+	}
+
+	public function testGetThumbnailReturnsNullForEmptyFile(): void {
+		$file = $this->createMock(File::class);
+		$file->expects($this->once())->method('getSize')->willReturn(0);
+
+		$this->appConfig->expects($this->never())->method('getPreviewConversionMaxFileSize');
+		$this->remoteService->expects($this->never())->method('convertFileTo');
+
+		$result = $this->provider->getThumbnail($file, 64, 64);
+
+		$this->assertNull($result);
+	}
+
+	public function testGetThumbnailAttemptsConversionWhenFileSizeIsExactlyAtLimit(): void {
+		$file = $this->createMock(File::class);
+		$file->expects($this->once())->method('getSize')->willReturn(100 * 1024 * 1024);
+
+		$this->appConfig->expects($this->once())
+			->method('getPreviewConversionMaxFileSize')
+			->willReturn(100 * 1024 * 1024);
+		// Conversion is attempted; throw to keep the test simple (image loading is not unit-tested here)
+		$this->remoteService->expects($this->once())
+			->method('convertFileTo')
+			->with($file, 'png')
+			->willThrowException(new \Exception('conversion failed'));
+
+		$result = $this->provider->getThumbnail($file, 64, 64);
+
+		$this->assertNull($result);
+	}
+
+	public function testGetThumbnailReturnsNullWhenConversionFails(): void {
+		$file = $this->createMock(File::class);
+		$file->expects($this->once())->method('getSize')->willReturn(1024);
+
+		$this->appConfig->expects($this->once())
+			->method('getPreviewConversionMaxFileSize')
+			->willReturn(100 * 1024 * 1024);
+		$this->remoteService->expects($this->once())
+			->method('convertFileTo')
+			->with($file, 'png')
+			->willThrowException(new \Exception('conversion failed'));
+
+		$result = $this->provider->getThumbnail($file, 64, 64);
+
+		$this->assertNull($result);
+	}
+}