My father has been sent tax documents from 2014 to 2019. What is the best and cheapest way to file all?

First---it is too late to receive a tax refund for any tax year earlier than 2016.  Refunds for 2014 and 2015 have been forfeited now.. But if tax is due for those years the IRS wants to be paid---with penalties and interest, which they will bill you for.   Those years can be prepared on paper by hand.

Software for 2014 and 2015 is no longer available.  You can still purchase and download the software for 2016, 2017, and 2018.  Those must each be prepared separately and mailed to the IRS in separate envelopes.  

The current online program can only be used for a 2019 return---no other year.

You cannot combine income from past tax years on the same return.  Each one is separate.

The other--cheaper-- way to file is by hand on paper forms which you have to mail in.

Online preparation and e-filing for 2018 is permanently closed.

To file a return for a prior tax year

If you need to prepare a return for 2016, 2017, or 2018 you can purchase and download desktop software to do it, then print, sign,  and mail the return(s)

https://turbotax.intuit.com/personal-taxes/past-years-products/

Remember to prepare your state return as well—if you live in a state that has a state income tax.

When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s.  Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.

Note:  The desktop software you need to prepare the prior year return must be installed/downloaded to a full PC or Mac.  It cannot be used on a mobile device.

If you are getting a refund, there is not a penalty for filing past the deadline.  If you owe taxes, the interest/penalties will be calculated by the IRS based on how much you owe and when they receive your return and payment. The IRS will bill you for this; it will not be calculated by TurboTax.

https://ttlc.intuit.com/questions/1900990-how-do-i-file-a-prior-year-tax-return

2014

Form 1040 https://www.irs.gov/pub/irs-prior/f1040--2014.pdf

Form 1040Instructions  https://www.irs.gov/pub/irs-prior/i1040gi--2014.pdf

Form1040A   https://www.irs.gov/pub/irs-prior/f1040a--2014.pdf

Form 1040A https://www.irs.gov/pub/irs-prior/i1040a--2014.pdf

Form1040EZhttps://www.irs.gov/pub/irs-prior/f1040ez--2014.pdf

Form1040EZ Instructions https://www.irs.gov/pub/irs-prior/i1040ez--2014.pdf

2015

Form 1040  https://www.irs.gov/pub/irs-prior/f1040--2015.pdf

Form1040 Instructions  https://www.irs.gov/pub/irs-prior/i1040gi--2015.pdf

Form1040A  https://www.irs.gov/pub/irs-prior/f1040a--2015.pdf

Form 1040A Instructions  https://www.irs.gov/pub/irs-prior/i1040a--2015.pdf

Form 1040EZ   https://www.irs.gov/pub/irs-prior/f1040ez--2015.pdf

2016

Form 1040  https://www.irs.gov/pub/irs-prior/f1040--2016.pdf

Form 1040 Instructions https://www.irs.gov/pub/irs-prior/i1040gi--2016.pdf

Form 1040A https://www.irs.gov/pub/irs-pdf/f1040a.pdf

Form 1040AInstructions https://www.irs.gov/pub/irs-prior/i1040a--2016.pdf

Form1040EZ https://www.irs.gov/pub/irs-prior/i1040gi--2016.pdf

2016

Form 1040  https://www.irs.gov/pub/irs-prior/f1040--2016.pdf

Form 1040 Instructions https://www.irs.gov/pub/irs-prior/i1040gi--2016.pdf

Form 1040A https://www.irs.gov/pub/irs-pdf/f1040a.pdf

Form 1040AInstructions https://www.irs.gov/pub/irs-prior/i1040a--2016.pdf

Form1040EZ https://www.irs.gov/pub/irs-prior/f1040ez--2015.pdf

Form1040EZ Instructions  https://www.irs.gov/pub/irs-pdf/i1040ez.pdf

https://www.irs.gov/filing/where-to-file-paper-tax-returns-with-or-without-a-payment

Form 1040EZ Instructions  https://www.irs.gov/pub/irs-pdf/i1040ez.pdf

2017

https://www.irs.gov/pub/irs-pdf/f1040.pdf

https://www.irs.gov/pub/irs-pdf/i1040gi.pdf

https://www.irs.gov/pub/irs-prior/f1040a--2017.pdf

https://www.irs.gov/pub/irs-prior/i1040a--2017.pdf

https://www.irs.gov/pub/irs-prior/f1040ez--2017.pdf

https://www.irs.gov/pub/irs-prior/i1040ez--2017.pdf

2018 Form 1040, Schedules and Instructions

https://www.irs.gov/pub/irs-prior/f1040--2018.pdf
 https://www.irs.gov/pub/irs-prior/i1040gi--2018.pdf
2018 Tax and EIC Tables https://www.irs.gov/pub/irs-prior/i1040tt--2018.pdf

IRS 2018 Schedules 1 thru 6 (only if needed)
Sch 1 : https://www.irs.gov/pub/irs-prior/f1040s1--2018.pdf
Sch 2 : https://www.irs.gov/pub/irs-prior/f1040s2--2018.pdf
Sch 3 : https://www.irs.gov/pub/irs-prior/f1040s3--2018.pdf
Sch 4 : https://www.irs.gov/pub/irs-prior/f1040s4--2018.pdf
Sch 5 : https://www.irs.gov/pub/irs-prior/f1040s5--2018.pdf
Sch 6 : https://www.irs.gov/pub/irs-prior/f1040s6--2018.pdf

https://www.irs.gov/filing/where-to-file-paper-tax-returns-with-or-without-a-payment

Remember to prepare your state returns as well—if you live in a state that has a state income tax.

https://ttlc.intuit.com/questions/1901670-how-do-i-contact-my-state-department-of-revenue