Below are several good commentaries. The problem is that due to their size, the files are very hard to navigate. For this reason we recommend either downloading the files and browsing them on a PC or installing a Bible study tool such as Xiphos or eSword.

TL;DR John Gill and Matthew Henry are our go-to commentaries, but the others are certainly worth mentioning.

• John Gill's Expositor PDF BibleHub
  • The PDF is a very large file (102 MB). Please use BibleHub or other tool when possible.
• Matthew Henry's Commentary PDF BibleHub
• Alexander Maclaren's Commentaries PDF BibleHub
  • Slightly easier to navigate in a browser or mobile device. 17 volumes.
  • Volume 1 - Genesis, Exodus, Leviticus And Numbers
  • Volume 2 - Deuteronomy, Joshua, Judges, Ruth, First Samuel, Second Samuel, First Kings, And Second Kings Chapters I To Vii
  • Volume 3 - Second Kings From Chap. Viii, And Chronicles, Ezra, And Nehemiah, Esther, Job, Proverbs And Ecclesiastes
  • Volume 4 - Psalm I To Cxlv
  • Volume 5 - Isaiah And Jeremiah
  • Volume 6 - Ezekiel, Daniel, And The Minor Prophets, Matthew I To Viii
  • Volume 7 - St. Matthew Chaps. Ix To Xxviii
  • Volume 8 - ST. MARK
  • Volume 9 - St. Luke
  • Volume 10 - John Chapters I. To Xiv
  • Volume 11 - St. John Chaps. Xv To Xxi
  • Volume 12 - The Acts
  • Volume 13 - Romans, Corinthians (To Ii Corinthians, Chap. V)
  • Volume 14 - Second Corinthians Chapter Vii To End, Galatians, Philippians, Colossians, Thessalonians, Timothy
  • Volume 15 – Philippians, Colossians, 1 & 2 Thessalonians, 1 & 2 Timothy, Titus
  • Volume 16 – Philemon, Hebrews, James, 1 Peter
  • Volume 17 – 2 Peter, 1, 2, 3 John, Jude, Revelation
• Matthew Poole's Commentary BibleHub
• John Owen's Commentary on Hebrews PDF
  • One of the best commentaries on Hebrews ever written.