EMAILS I send to people end up in their SPAM box

EMAILS  which appear in other peoples SPAM are outside of your control. You can not alter their spam rules on their server or personal workstations. They would need to make sure that their SPAM filters are not causing false positives or they need to add you to their whitelist. 

They can also provide you with a list showing what triggered your email to their spam. If you sent just a PDF or FILE, ALL CAPS, or no message body, this can cause false positives. 

But there is no way for anyone but them to tell you what triggered it on their end.