How do I import 1099-B for a (Previous Year) from either CSV or PDF file into Turbotax? Broker import only works for the current tax year and I have 1000+ transactions

The only file format that TurboTax will import is .TXF, but you don't have to import all the transactions. You can make summary entries in TurboTax, and attach a statement to your tax return giving the details of the transactions. You can create the statement from whatever file you have.

In TurboTax, on the screen that asks if you got a 1099-B or a brokerage statement, answer Yes. On the screen that asks how you want to enter your Form 1099-B, select "I'll type it in myself." On the next screen, enter the name of the brokerage. You will then get a screen with two choices: "I'll enter one sale at a time" or "I'll enter a summary for each sales category." Select the second option, and it will then give you boxes to enter the total proceeds and total cost, and select the sale category (short-term or long-term, covered or noncovered). After that screen, you will get a screen that asks if you need to enter another sale, meaning totals for another category. You have to enter separate totals for each category.

The statement that you attach to your return has to contain the same information as Form 8949, in a similar format. The information needed is a description of the property sold, dates of acquisition and disposition, proceeds, basis, adjustment and code(s) if applicable, and gain or loss. Put your name and Social Security number at the top of each page of the statement. If you have a brokerage statement that lists all the transactions for the year, you can probably just use a copy of the brokerage statement as the attached statement. Attach the statement behind all the IRS forms and schedules.