If I am you. I would do like below:
1) put the pdf somewhere on your website. (like : www.yourwebsite.com/pdf/thepdf_file.pdf
2) save your 1st page of pdf file as jpg(eg. thepdf_file.jpg) you can also put a wording say "click me to get the detail".
3) add the thepdf_file.jpg file and link to "www.yourwebsite.com/pdf/thepdf_file.pdf"
once when the recepient receives your email...it shows the pdf cover page and directs he/she to the download link when the jpg was clicked.
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