To open S3 objects directly in the browser instead of downloading them, you'll need to make some adjustments to your setup. Here's how you can modify your PHP code and S3/CloudFront configuration to achieve this:

1. Set the correct Content-Type header for your S3 objects:
   Make sure your S3 objects have the appropriate Content-Type metadata set. For example, PDFs should have "application/pdf", images should have "image/jpeg" or "image/png", etc.

2. Update your CloudFront distribution:
   Ensure your CloudFront distribution is configured to forward the Content-Type header from S3 to the browser.

3. Modify your PHP code:
   Instead of using a direct link to the S3 object, you can create a PHP script that serves the file with the correct headers. Here's an example:

```php
<?php
require 'vendor/autoload.php'; // Include the AWS SDK for PHP

use Aws\S3\S3Client;
use Aws\Exception\AwsException;

// Configure AWS credentials and region
$s3 = new S3Client([
    'version' => 'latest',
    'region'  => 'your-region',
    'credentials' => [
        'key'    => 'your-access-key-id',
        'secret' => 'your-secret-access-key',
    ],
]);

$bucket = 'your-bucket-name';
$key = 'path/to/your/file.pdf'; // The S3 object key

try {
    // Get the S3 object
    $result = $s3->getObject([
        'Bucket' => $bucket,
        'Key'    => $key
    ]);

    // Set the appropriate headers
    header("Content-Type: {$result['ContentType']}");
    header("Content-Length: {$result['ContentLength']}");
    header('Content-Disposition: inline; filename="' . basename($key) . '"');

    // Output the file contents
    echo $result['Body'];
} catch (AwsException $e) {
    // Handle any errors
    echo "Error: " . $e->getMessage();
}
```

4. Update your HTML links:
   Instead of linking directly to the S3 URL, link to your PHP script:

```html
<a href="view_file.php?key=path/to/your/file.pdf">View File</a>
```

5. Create a view_file.php script:
   This script will handle the file serving:

```php
<?php
// Get the file key from the URL parameter
$key = $_GET['key'];

// Include the file serving code from step 3 here
// Make sure to validate and sanitize the $key parameter to prevent security issues
```

By following these steps, your files should open in the browser instead of downloading. The PHP script acts as a proxy, fetching the file from S3 and serving it to the browser with the correct headers.

Remember to handle security properly, such as validating user permissions and sanitizing input parameters. Also, ensure your CloudFront distribution is configured correctly to work with this setup.

Would you like me to explain any part of this solution in more detail?