Users having problems sending PDF files

We have customers who send us print files, they say recently that the PDF section doesn't work.

www.treehousestickers.com

Thank you for contacting us.

The PDF extension is not blocked on your form, and I was able to upload a test PDF file via your website. If there are any specific steps to reproduce the issue, please let us know. Also, any additional information would be appreciated, for example if this is related only to some browsers or to some file size.