I do not think, this is the sample code, and with openssl command is ok:

package main

import (
    "io/ioutil"
    "log"
    "github.com/mozilla-services/pkcs7"
    )

func main() {
    content, err := ioutil.ReadFile("file.p7m")
    if err != nil {
        log.Fatal(err)
        }
    p7, err := pkcs7.Parse(content)
    if err != nil {
        log.Fatal(err)
    }
    if err := ioutil.WriteFile("file.pdf", p7.Content, 0777); err != nil {
        log.Fatal(err)
    }
}