If you are trying to create scalable stamps you may have to save as PNG and use an outside source to bring them into a PDF. I just did it and maintained transparency using Nitro pro. There is nothing in the Save as (PDF) options to allow for transparency nor color depth Nitro has a neat feature that allows me to extract all the images from a PDF. I believe their free reader has that too (Which made it hand for testing the transparency as even in Nitro, the transparent background shows white )
I've tried doing this in the past without success.
I don't believe SnagIt has ever exported to PDF with transparency. I tried to force it to do so today without any luck.
I even installed SnagIt 10 and gave it a Try. No luck.I would have installed a newer version. But the installation files tend to install themselves in the SnagIt 2018 install folder and screw everything up. Plus, it was good to look back that far into the history of PDF's and SnagIt. As soon as I switched to save as a PDF. The "Save with transparency" check box "Gray's Out" and the preview window loses the transparency grid. Typical PNG Save shown here.