is templating only for e-mails or is it also possible to edit the pdf templates for invoices?
The templates you mentioned are the pdf templates. eMail templates are available via the settings: 
for the pdf template, if I would like to modify the item name “Invoice Date” to for example “Purchase Date”, any suggestion where I should look at?
If you just need to change the label from “Invoice Date” to something else, you simply need to modify the related language file. You can edit the custom_lang.php file located in application/language/[your language] folder and add this row:
'invoice_date' => 'Purchase Date',
If this is the first time you edit this file, the code should look like:
$lang = array(
'invoice_date' => 'Purchase Date',
);
This will override the default translation label from “Invoice Date” to “Purchase Date”