If you moved to a server with PHP 5.4, you might get blank invoices from your admin when using the Print Invoice button.

If you Googled this, you probably found yet another StackOverflow link – http://stackoverflow.com/questions/17022829/magento-blank-empty-page-whene-printing-invoice-on-backend

From the above link:

Make a core override:

Copy – [magento root]/lib/Zend/Pdf/FileParserDataSource.php

To – [magento root]/app/code/local/Zend/Pdf/FileParserDataSource.php

Fixed it by commenting out __construct() and __destruct() methods in lib/Zend/Pdf/FileParserDataSource.php

// abstract public function __construct();

/**
* Object destructor. Closes the data source.
*
* May also perform cleanup tasks such as deleting temporary files.
*/
// abstract public function __destruct();

Then clear Magento cache and test.