If you don't have a mass emailing tool, you can still create a mail merge campaign using Outlook's mail merge feature with the help of a few other tools in the office suite.
To create your campaign, first compose your email message in Word and create an Excel file to contain the fields that you'll need merged.
In your Excel file:
Add a new column that will contain the email addresses of your recipients (as many as you'd like). Then add another column with your DocSend link you want to send (ex. https://docsend.com/view/abc123).
Next, create a third column that is simply (1) and (2) combined together in a formula: =CONCATENATE([DocSend link column], "?email=", [email address column])
This should produce a custom link for each email address, such as https://firstname.lastname@example.org
Now you're all set from an Excel perspective!
In your Word document:
Select the Mailings tab in the upper navigation bar. From there go to the Select Recipients menu and choose Use an Existing List.
From your documents, select the Excel file you just created and allow for MSFT Word to access your information.
You'll then want to Insert the merge fields from your Excel sheet. You can see I've inserted the <<Name>> field and <<DocSend_Link>> field below:
Once you've previewed the merge and are happy with your email, select Finish and Merge then Merge to Email for your email to send to the addresses from your excel sheet.
If you'd like to create a hyperlink for your DocSend link merge field, follow the steps listed here. Note that this will give you a named link for each message, i.e. instead of inserting the link directly, your email can contain a link with friendly text like "Click Here".