How do I load a PDF of a Form 1099B?

you can't. imports can be done from accounting software - a TXF file and certain versions of Quicken and Quickbooks or from a financial institution that partners with TurboTax.

you're not out of luck you would need to enter summary totals in a 1099-B worksheet. then file form 8453 with 8949 box check and send the form with a PDF of the 1099-B 

If you are filing your tax return using an online provider,
mail Form 8453 to the IRS within 3 business days after
you have received acknowledgement from your
intermediate service provider and/or transmitter that the
IRS has accepted your electronically filed tax return. If
you don’t receive an acknowledgement, you must contact
your intermediate service provider and/or transmitter.
Mail Form 8453 with 1099-B to:
Internal Revenue Service
Attn: Shipping and Receiving, 0254
Receipt and Control Branch
Austin, TX 73344-0254