in helpers.php a function[PDFavailable] will be called, result should be true or false.

But the necessary file[dompdf_config.inc.php] does not exists in directory[components/com_invoices/helpers/dompdf/]!
Maybe other source code is also missing.
File or code is also not available in invoicemanager_install/component zip

In description of invoice manager (marketing of homepage) this feature is included but not delivered

Here the function out of your helpers.php

static function PDFavailable(){
$version = PHP_VERSION >= 5.0 ? true : false ;
$quotes = get_magic_quotes_gpc();

if(file_exists(JPATH_SITE.DS.'components'.DS.'com_invoices'.DS.'helpers'.DS.'dompdf'.DS.'dompdf_config.inc.php') && $version && !$quotes){
return true;
}
else return false;

}
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