Not able to view attachments in apple mail

If the messages have been send with Mail Drop, then a 30 day download window applies. Older messages will not be able to retrieve the attachment anymore. Other limits may apply.

See this article Send attachments with your Gmail message - Computer - Gmail Help

The pdf file that is sent by another person could be corrupted , using Mac OS high Sierra you must use a feature optimise storage How to free up storage space on your Mac - Apple Support

You have to click on download attachments see this screen shot , right now it is not showing as I have clicked on the option to download attachments , also there must be a space in Hard disk , if that is exceeding the limit that could be a reason .

These boxes are to be checked and have a green checkmark .

It depends upon the compatibility what app your are using in your Mac to download attachments .

An example if a sender sends an attachment in auto cad format and if I am not having latest version of auto cad and it is outdated , the file will not open , same way with pdf files , you need third party applications as adobe .

You have to call the sender on phone or mail them to send the attachment with mail again .

The receiver has no control over it .

And you must use reduce clutter free options and unnecessary old mails / mailboxes that are of very less use should be deleted ( keep the data in limits ) .

A method is there how to see the mail data , open mail application , right click over inbox and click on get account info .

For gmail accounts the maximum limit is 18.25 GB .

In safari preferences > general the box for open safe files after downloading is kept as unchecked , because that is not sure file like pdf could be corrupted or malicious as it is coming from outsource , in your case you must call them and tell them to send it once again or the file size could be enormous , the file download location - should be download if you click on drop down arrow of it select ask for each download , it will stop and ask permissions , then you have to enter admin name and password and if you select other you have to decide or choose at which location the file should get downloaded .