It probably was an automatic table before it was converted to pdf. See if you can find the original document. Documents coverted or opened from pdf or any other format can be a real pain to edit.
Strategy when importing from another format:
My general strategy when I get something in a format other than Word is to copy and paste into a new Word document as plain text and then format in that document using styles. I use the original for a reference. I've been working with Word for more that 25 years and find that this method saves me time and headaches.
Generating a Table of Contents - an Automatic Table
Here are three references:
I am not going to repeat everything in those references here. Creating and updating a TOC in Word is generally very simple, but it can get complex. The simple method involves use of the built-in heading styles to mark the headings you want in the TOC. (You can modify the appearace of these styles.) Then insert a Table of Contents from the References tab. Be careful when applying the heading styles. Do not select text but rather just click in the heading and apply the style to the entire paragraph.
Importance of Styles in Word
Basic Concepts of Microsoft Word - from Shauna Kelly