Scammers frequently send out emails which appear to come from perfectly reputable companies, with attachments (such as invoices) accompanying them.
In some cases they're trying to get payments for non-existent services sent to them but in others they're simply trying to get inquisitive people to open the attachments so that some malware can be dumped their computers.
So, if you actually clicked on the attachment to open it, you MUST now do a full scan with your anti-virus program AND a scan with the free version of Malwarebytes Anti-Malware:
https://www.malwarebytes.org/antimalware/
(NB: When installing Malwarebytes, ensure that you remove the tick alongside the pre-ticked option to take a free trial of the Premium version).
For example, I recently got an email purporting to have an invoice related to my account for Dartford Crossing payments. I was tired when I got it (OK, let's admit it, I'd actually just got back from the pub!) and clicked to open the attachment before realising that the email hadn't been sent to the address I've got registered to my Dartford Crossing account. I immediately ran those scans and discovered that it had dumped malware onto my computer which, if it had gone unspotted, could have caused me further problems later on. (I quickly zapped it though).
If you want to report a scam email, go here:
http://www.actionfraud.police.uk/scam-emails
If you want to tell Gentiva (which is a perfectly respectable company) that their company details are being abused, send an email to:
[email protected]