Dumdum
Files in general can't be attached to a SMS message unless they are photos or as you say maybe videos. You will notice that even full resolution photos will be downsized when sent as a SMS. You may be used to attaching other kinds of files with other messaging services, such as RCS (which is a "standard" that is gradually replacing SMS on Android and now iPhone).
I put "standard" in quotes because, as I understand it, RCS is actually proprietary, meaning that there is no Free/Open Source Software implementation available now or likely for the future. The GrapheneOS messaging app dues not support it, nor do other F/OSS text messaging apps.
So unfortunately, I believe this is a way in which the world of "texting" is leaving those of us who prefer to avoid corporate software behind. Alternatives would be putting the PDF online somewhere and texting a link to it, exporting or screenshotting the PDF into a format like PNG or JPG that can be texted, or sending by email or by a messaging app like Signal. Or, you could install a proprietary texting app like Google Messages or Facebook Messenger on your GrapheneOS phone.