Hey there @userX9naUA4Eli,
You would click the Four Square menu to access the feature.
Like you I don't have a + button, but I have found a work around.
Go to your gallery and select the picture, then the share button, then messages.
You then type the name of the person and a message opens with the picture in it.
PDF files are incompatible with either SMS or MMS messaging due to Adobe formatting restrictions. There are third party apps that use alternate data channels to accomplish this, but sending a PDF is easiest using email.
Just to make sure I understand... sending a PDF is not an option using the Samsung messaging app? I'm unable to send a PDF on my Note 9. I downloaded Google messenger and Textra, and neither were able to add the attachment either. However, when I use my Pixel 2 XL, any of the texting apps DO have the option to send a PDF. So my conclusion is that the Note 9 (and other Samsung devices) do not have the capability using any texting app, meaning it's a device deficiency and not an app issue.