The short answer is:
You are correct that there is no native integration between HubSpot and Flex Rental Solutions to seamlessly pull quote PDFs or links into HubSpot emails, nor does Zapier offer a pre-built connector for that specific action, meaning the manual attachment is the immediate solution.
However, since Flex Rental Solutions does have a REST API and HubSpot has a robust API, the best long-term solution is to build a custom application, or “Private App,” to automate this.
This custom connector would automatically monitor Flex for new quotes, download the quote data or PDF link, and push it directly into the relevant HubSpot Deal record, which can then be used in an email template via a personalization token, making the process seamless 
The long answer is:
The reason you don’t see a seamless integration is that Flex Rental Solutions is niche inventory management software, and while HubSpot integrates with thousands of systems, deep operational integrations like this usually require custom work, as noted by another user in the thread.
The good news is that the core components for a world-class solution are readily available 
Since Flex Rental Solutions offers a REST API (called the Flex5 API), and HubSpot offers the HubSpot API (specifically the CRM Objects and Files APIs), you can build a stable, customized data pipeline.
This pipeline would typically run on a secure, external platform like Google Cloud Platform or a simplified container platform like Stape that acts as a middleware server.
Here’s how the superior solution works: the custom app continuously pings the Flex Rental Solutions API to check for newly created or updated quotes.
When a new quote is detected, the app retrieves the data, including the link to the generated quote PDF.
It then uses the HubSpot API to locate the corresponding Deal or Contact record.
Finally, the app pushes the quote’s URL into a custom property on the HubSpot Deal object.
This process is far better than simply attaching a PDF manually because it enables automation: you can now use a HubSpot workflow to trigger an email to the client immediately after the quote is created in Flex, and the email can use a personalization token to include the direct, current quote URL, eliminating manual file management and ensuring data accuracy across both systems 
This API-driven approach ensures high data integrity, scales with your business, and provides the best possible customer experience.
This comment was generated with the assistance of an AI tool, incorporating my expertise in integrations 