I've tried to check the signature that should be embedded in an invoice pdf with
pdfsig (poppler v. 0.76.1-1) and
masterpdfeditor5 (v. 5.4.10) on Arch Linux (5.1.5-arch1-2-ARCH), but neither of them detects the signature.
According to the pdf's "producer" metadata, it was generated with
Altsoft XML2PDF. The original XML file was provided. According to a guide from the company that send the invoice, a banner should appear in Adobe Reader allowing to import the cert and verify the signature.
Is there a program that can be installed on my system without much difficulty other than Adobe Reader? I prefer FLOSS licenses.