https://www.whofic.com/
By far the largest dedicated Doctor Who archive on the web is A Teaspoon and an Open Mind (often referred to as "Teaspoon"). This archive as I type has 49,965 fics.
Note that there are two ways of viewing stories on this site: the first is chaptered and the second is an all-chapter view.
https://www.whofic.com/viewstory.php?sid=*
https://www.whofic.com/viewstory.php?ac ... hapter=all
* marks the story ID in the web address.