وَلَوْ شِئْنَا لَبَعَثْنَا فِى كُلِّ قَرْيَةٍ نَّذِيرًا · فَلَا تُطِعِ ٱلْكَٰفِرِينَ وَجَٰهِدْهُم بِهِۦ جِهَادًا كَبِيرًا
"Had We willed, We would have sent a warner to every town. So do not obey those who disbelieve, and strive against them with it, with a great striving."
The lexicographers generally give the Qurʾān as the referent of the pronoun — and indeed its nearest referent is the book that has been spoken of since the thirty-second verse.
I record this as an observation, and its ground is that a pronoun goes to its nearest referent: what is shown as the instrument of the struggle is the text itself. And the qualifier is kept large: jihādan kabīrā.
The root ج-ه-د was treated at al-ʿAnkabūt 29/6 and 29/69 (to expend strength, to go to the end of one's capacity) — it was set out in a table there that the root binds the two ends of that sūra. I rely on that treatment.
And its connection with the fifty-first verse is worth recording: that a separate warner was not sent to every town enlarges the scope of a single warner's effort. Read together, the two verses make the ground for the qualifier "a great striving" visible. I give this as my own reading.