Having inspected your PDF file the first guess in the comments to your question has been confirmed...
Your sample document is scanned and essentially consists of one bitmap image per page. When you zoom into the document, you can quickly see that all content looks fairly pixel'ish.
All the images have a resolution of 2600x3799 and are black and white.
These images have furthermore been OCR'ed and the resulting text has been invisibly added to the pages which allows for selecting, copying & pasting.
E.g. have a look at the top of page 885:
Its content stream starts like this:
1 0 0 1 -0.5998 -0.4801 cm
1 1 1 rg
1 i
/RelativeColorimetric ri
/GS0 gs
0 0 469.2 684.7 re
f
q
467.9972 0 0 683.8015 0.6014 0.4492 cm
/Im0 Do
Q
Here /Im0, the page image, is inserted
1 0 0 1 0.5998 0.4801 cm
0 0 0 rg
BT
/TT0 1 Tf
3 Tr 9.8 0 0 10.4 35.8002 640.4199 Tm
Here addition of text is prepared; especially have a look at 3 Tr
: This oparation sets the text rendering mode to 3
which is Neither fill nor stroke text (invisible). (section 9.3.6 Text Rendering Mode in ISO 32000-1:2008)
(A )Tj
/TT1 1 Tf
-0.01 Tc 8.8 0 0 9.5 43.4002 640.4199 Tm
(%gust )Tj
Here you see text added, starting with an 'A ' and an '%gust '. This actually shows that the result of the OCR'ing does not seem to have been properly checked as that should have been 'August'. The low quality text information continues:
A %gust , 1978 SHORT PAPERS 885
where
and also
Similarly for B. Also,
T, = AY-l T
as a result of the adiabatic cooling of the vapour.
Stage 2:
Here a volume of vapour and a volume of liquid I are removed and replaced with an
equal volume of air containing concentrations Y and s of A and B, respectively. Of course,
r or s may either or both be negligibly small, with subsequent simplification.
As you see many special characters and formulas have not or not correctly been recognized.