You didn’t indicate whether you can understand replies in English, so I’ll hope that you can!
Let’s Encrypt certificates can definitely be converted to PFX format, but it’s not an encouraging sign that your hosting provider asked you to send a certificate file at all. Let’s Encrypt is designed around the concept of automation and discourages people from obtaining certificates manually. In connection with this, the lifetime of an individual Let’s Encrypt certificate is only 90 days, after which it must be replaced with a new certificate.
The recommended way to use Let’s Encrypt is to have the hosting provider use software to request certificates automatically, without human intervention. I would suggest asking your hosting provider to add Let’s Encrypt support within the hosting environment, or else switching to a different hosting provider that already supports Let’s Encrypt, or else paying for a 1-year paid certificate from a paid certificate authority. I don’t think you’ll have a good experience frequently obtaining Let’s Encrypt certificates manually and converting them to a different format.
Please let us know if you require a reply in French, or if you are still convinced that you want to proceed with using Let’s Encrypt in a manual way even though it may involve unnecessary effort.