In phpdocx 10 i have the issue then i make tranformDocument with the same names of transformFiles (like example.docx,example.pdf)
it's a code from vendor/phpdocx/Classes/Phpdocx/Transform/TransformDocAdvLibreOffice.php:174 - i have the rename the same file with the same extensions
// get the converted document, this is the name of the source and the extension
$newDocumentPath = $outdir . '/' . $sourceFileInfo['filename'] . '.' . $filesExtensions['targetExtension'];
// move the document to the guessed destination
rename($newDocumentPath, $target);