Do you mean the form results attached as a PDF or do you mean a submitted PDF file (uploaded as part of the form submission) is being blocked - they are two different matters.
I would think it's probably a GMail issue if any sort of attachment from Machform is being blocked from being opened by the recipient - it may even be some policy or term of use for GMail accounts or it might be some setting your Gmail user needs to tweak to allow attachments. Are any other file-types blocked that are submitted as attachments?
If it is Gmail itself, can you try sending the form submissions to another type of email e.g. one created on your domain name, through your hosting account - in cPanel, you usually have the ability to set up email accounts. See if one of those would work.
Machform already has the ability to send the link to the PDF (submitted as part of the form submission) instead of attachment. I would guess the option is either in the 'upload file' field properties, or the form's properties to either send the upload as a file or provide the link.
It might be worth asking Appnitro directly - you need to open a support ticket from your customer area.
Might be worth considering, or deciding you don't need the PDF version of the submission, and just have the email confirmations set to the users.